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123
Image-Guided
High-Precision
Radiotherapy
Esther G. C. Troost
Editor

Image-Guided High-Precision Radiotherapy

Esther G. C. Troost
Editor
Image-Guided
High-Precision
Radiotherapy

Editor
Esther G. C. Troost, MD, PhD
Department of Radiotherapy and Radiation Oncology
Faculty of Medicine and University Hospital Carl Gustav Carus
Dresden, Sachsen, Germany
ISBN 978-3-031-08600-7 ISBN 978-3-031-08601-4 (eBook)
https://doi.org/10.1007/978-3-031-08601-4
© Springer Nature Switzerland AG 2022
This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of
the material is concerned, specically the rights of translation, reprinting, reuse of illustrations, recitation,
broadcasting, reproduction on microlms or in any other physical way, and transmission or information
storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology
now known or hereafter developed.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication
does not imply, even in the absence of a specic statement, that such names are exempt from the relevant
protective laws and regulations and therefore free for general use.
The publisher, the authors, and the editors are safe to assume that the advice and information in this book
are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the
editors give a warranty, expressed or implied, with respect to the material contained herein or for any
errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional
claims in published maps and institutional afliations.
This Springer imprint is published by the registered company Springer Nature Switzerland AG
The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

v
Preface
Radiation therapy is one of the pillars of oncological treatment. As opposed to sur-
gery, external beam radiation therapy requires the indirect depiction of the tumour
and its surrounding structures both during the phase of treatment planning and dur-
ing fractionated treatment. Historically, only pretreatment imaging was available
and served as basis for the entire course of treatment, irrespective of anatomical
changes caused, e.g. by tumour response or by patient’s weight loss.
During the last 15 years, advances in the eld of image-guided radiotherapy have
been dramatic. Anatomical and functional imaging is available prior to and during
the course of treatment, occasionally even during the treatment fraction. Novel,
tumour type-specic radionuclides have been developed for positron emission
tomography (PET) and enable depiction of small tumour deposits, which would
otherwise have been overlooked. Fast, highly precise radiation therapy techniques
enable the treatment of small lesions. Linear accelerators integrated with magnetic
resonance imaging (MRI) have revolutionized the eld since they facilitate online
real-time image-guided radiation dose delivery of moving soft-tissue targets.
Herewith, safety margins compensating for repeat patient positioning and target
motion can be reduced or even abolished, thus reducing dose to normal tissues and
hopefully subsequent side effects.
This book provides the reader with an overview of the value of PET with widely
available as well as more exclusive, tumour-specic for radiation treatment plan-
ning. In-room equipment for online positioning on the linear accelerator is summa-
rized and radiation treatment techniques relying on those imaging possibilities are
explained to experts outside the eld of radiotherapy. Moreover, the value of MRI
for soft-tissue tumours both during the phase of target volume delineation for treat-
ment planning as well as during MR-LINAC treatments is focused on. Brachytherapy
of several tumours, such as prostate and gynaecological tumours, heavily depends
on MRI, also this is exemplied in one of the chapters. The ample possibilities of
ultrasonography for image-guidance are furthermore referred to. In those tumours
not well visible on imaging, the use of ducial markers may play a role?this is
described for oesophageal and prostate cancer. Lastly, the use of articial intelli-
gence, multimodal imaging for prediction of tumour control as well as of normal-
tissue side effects are topics of the remaining three chapters.

vi
Together with the authors of the different chapters, I hope that this book will be
of value to residents and senior physicians in the elds of radiotherapy, radiology,
and nuclear, as well as to the physicists and radiation technologists of the respective
disciplines.
Dresden, Sachsen, Germany Esther G. C. Troost
April 2022
Preface

vii
Part I T
1 Use of [
18
F]FDG PET/CT for Target Volume Definition in
Radiotherapy�� 3
Hanneke E. E. Pouw, Dennis Vriens, Floris H. P. van Velden,
and Lioe-Fee de Geus-Oei
2 Specific PET Tracers for Solid Tumors and for Definition of the
Biological Target Volume�� 31
Constantin Lapa, Ken Herrmann, and Esther G. C. Troost
3 Use of Anatomical and Functional MRI in Radiation Treatment
Planning�� 55
Angela Romano, Luca Boldrini, Antonio Piras,
and Vincenzo Valentini
Part II Image-Guided Radiation Therapy Techniques
4 In-Room Systems for Patient Positioning and Motion Control�� 91
Patrick Wohlfahrt and Sonja Schellhammer
5 IMRT/VMAT-SABR��109
Pablo Carrasco de Fez and Núria Jornet
6 Magnetic Resonance-Guided Adaptive Radiotherapy: Technical
Concepts��135
Sara Hackett, Bram van Asselen, Marielle Philippens,
Simon Woodings, and Jochem Wolthaus
7 MR-Integrated Linear Accelerators: First Clinical Results��159
Olga Pen, Borna Maraghechi, Lauren Henke,
and Olga Green
8 Image-Guided Adaptive Brachytherapy��179
Bradley Pieters and Taran Paulsen-Hellebust
Contents

viii
9 Ultrasonography in Image-Guided Radiotherapy:
Current Status and Future Challenges��201
Davide Fontanarosa, Emma Harris, Alex Grimwood, Saskia Camps,
Maria Antico, Erika Cavanagh, and Chris Edwards
10 Means for Target Volume Delineation and Stabilisation:
Fiducial Markers, Balloons and Others��221
Ben G. L. Vanneste, Oleksandr Boychak, Marianne Nordsmark,
and Lone Hoffmann
11 Artificial Intelligence in Radiation Oncology: A Rapidly Evolving
Picture��249
Harini Veeraraghavan and Joseph O. Deasy
Part III Outcome Evaluation
12 Multi-Modality Imaging for Prediction of Tumor Control
Following Radiotherapy��271
Daniela Thorwarth
13 Modelling for Radiation Treatment Outcome��285
Almut Dutz, Alex Zwanenburg, Johannes A. Langendijk,
and Steffen Löck
Contents

Part I
Target Volume Definition

3
1
Use of [
18
F]FDG PET/CT for Target
Volume Definition in Radiotherapy
Hanneke E. E. Pouw, Dennis Vriens, Floris H. P. van Velden,
and Lioe-Fee de Geus-Oei
Abbreviations
ATLAAS Automatic decision Tree-based Learning Algorithm for Advanced
Segmentation
ATP Adenosine-5′-TriPhosphate
CT X-ray Computed Tomography
CTAC (low dose) CT performed for Attenuation- and scatter Correction of
the PET-image
H. E. E. Pouw (*)
Section of Nuclear Medicine, Department of Radiology, Leiden University Medical Center
(LUMC), Leiden, The Netherlands
HollandPTC, Delft, The Netherlands
Department of Medical Oncology, Amsterdam UMC location Vrije Universiteit Amsterdam,
Amsterdam, The Netherlands
e-mail: [email protected]
D. Vriens
Section of Nuclear Medicine, Department of Radiology, Leiden University Medical Center
(LUMC), Leiden, The Netherlands
HollandPTC, Delft, The Netherlands
e-mail: [email protected]
F. H. P. van Velden
Section of Nuclear Medicine, Department of Radiology, Leiden University Medical Center
(LUMC), Leiden, The Netherlands
e-mail: [email protected]
L.-F. de Geus-Oei
Section of Nuclear Medicine, Department of Radiology, Leiden University Medical Center
(LUMC), Leiden, The Netherlands
Biomedical Photonic Imaging Group, University of Twente, Enschede, The Netherlands
e-mail: [email protected]
© Springer Nature Switzerland AG 2022
E. G. C. Troost (ed.), Image-Guided High-Precision Radiotherapy,
https://doi.org/10.1007/978-3-031-08601-4_1

4
CT-TV CT-only based Target Volume
CTV Clinical Target Volume
EANM European Association of Nuclear Medicine
EARL EANM Research Ltd.
EORTC European Organisation for Research and Treatment of Cancer
ESTRO European SocieTy for Radiotherapy and Oncology
FBP Filtered Backprojection Reconstruction Algorithm
[
18
F]FDG 2-[
18
F] uoro-2-deoxy-d-Glucose
GTV Gross Tumour Volume
HNSCC Head-and-Neck Squamous Cell Carcinoma
LOR Line-Of-Response
MRI Magnetic Resonance Imaging
NSCLC Non-Small Cell Lung Carcinoma
OAR Organs At Risk
OSEM Ordered Subsets Expectation Maximisation reconstruction
algorithm
PET Positron Emission Tomography
PET-AS Automatic Segmentation of contours in PET
PET-TV PET(/CT)-based Target Volume
Planning-CT (High dose, contrast-enhanced) CT performed for radio therapy
treatment planning purpose
PTV Planning Target Volume
STAPLE Simultaneous Truth and Performance Level Estimate
SUV Standardised Uptake Value
SCLC Small Cell Lung Carcinoma
1.1 Introduction
High-precision radiotherapy is of increasing importance in oncological treatment,
by striving for higher effectiveness and decreasing side effects by more conformal
dose delivery. This can be accomplished by new X-ray photon radiotherapy tech-
niques, including volumetric-modulated arc therapy (Chap. 5) and intrafraction
adaptation using the magnetic resonance-based linear accelerator (Chaps. 6 and 7),
by particle therapy (protons, carbon ions; Chap. 10), or by highly precise target
volume denition, which will be the focus of the current chapter.
Different phases can be distinguished in the development of a treatment plan.
First, accurate information about the localisation and extension of the tumour vol-
ume, with respect to its surroundings, should be obtained, the so-called gross tumour
volume (GTV). This information is obtained by a combination of different diagnos-
tic methods, including physical examination (visual inspection, palpation), medical
imaging [endoscopy, (endo)ultrasonography, computed tomography (CT), mag-
netic resonance imaging (MRI), and positron emission tomography (PET)], and
cytology or histopathology. Second, the information of these different modalities
needs to be properly co-registered to the CT scan used for treatment planning pur-
poses. Third, the target volumes are dened on the planning CT: GTV is dened as
H. E. E. Pouw et al.

5
the tumour visible (or palpable) on physical examination or imaging. The clinical
target volume (CTV) takes into account the microscopic tumour extension and the
planning target volume (PTV) includes additionally systematic and random setup
inaccuracies, resulting in the volume to be irradiated. These margins make the plan-
ning more reliable, but also account for a smaller therapeutic window [1]. The de-
nitions of the target volumes are dened in International Commission on Radiation
Units and Measurements (ICRU) Report 50 [2]. Also, the organs at risk (OAR), in
which the radiation dose is to be maximally reduced, are segmented on the images.
Finally, a treatment plan is generated, taking into account geometric (e.g. motion,
patient setup-variability) and physical (e.g. particle range) uncertainties.
Image contrast in the planning-CT, which is ideally acquired in radiotherapy
position, is determined by differences in photon attenuation of various tissues and
thereby delivers anatomical (geometrical) information. The (semi)quantitative
information on photon attenuation can be used to estimate the behaviour of high-
energy photons in human tissues and thus to reliably calculate radiation treatment
plans. CT, however, suffers from limited soft-tissue contrast, which can for a limited
extent be overcome by the addition of intravenous iodinated contrast media.
Additional imaging by other imaging modalities can assist in more accurate seg-
mentation of the target volume. MRI is based on a different physical property of
tissues: its nuclear magnetic spin resonance. Conventional MRI-sequences result in
anatomical images of higher soft tissue contrast, compared to CT, but might, just as
CT, be awed by artefacts including magnetic susceptibility (metal implants), geo-
metrical distortions, motion and ow (perfusion).
PET provides biological information of the tumours and organs at risk, which
can be co-registered to the anatomical domain of CT or MRI. Current standard-of-
care is the combination of PET with CT. Although PET/MRI is currently available,
clinical application in radiotherapy planning is not yet widely implemented due to
technical challenges, such as accurate MRI-based attenuation correction of the PET-
images (see later) and geometrical distortions of the MRI [3–5]. Which biological
information is obtained by PET-imaging depends on the administered radiopharma-
ceutical (“tracer”). This chapter will cover the use of the most widely used PET-
radiopharmaceutical 2-[
18
F] uoro-2-deoxy-d-glucose ([
18
F]FDG). The usage of this
specic radiopharmaceutical can be assumed when PET is mentioned in the remain-
der of this chapter. Chapter 2 will cover the use of PET tracers beyond [
18
F]FDG.
To gain insight into the additional value of PET in target volume delineation
(TVD), rst, the basic principles of PET/CT and the radiopharmaceutical [
18
F]FDG
will be discussed, followed by the image acquisition and reconstruction techniques,
including methods for motion control (“Technical Aspects”). The role of [
18
F]FDG
PET/CT in tumour volume and lymph node segmentation, together with its value in
different disease sites will be discussed in ?Target volume delineation?. The in u-
ence of the applied segmentation method is discussed in “PET/CT segmentation
methods” and the risks and disadvantages of the application of PET will be high-
lighted in section “Cons and pitfalls”. The chapter will end with a future perspective
on the role of [
18
F]FDG PET/CT in treatment planning.
Out of the scope of this chapter is the established role of [
18
F]FDG PET/CT in
determining the extent of different oncological diseases (staging), the use of
1 Use of [
18
F]FDG PET/CT for Target Volume Denition in Radiotherapy

6
[
18
F]FDG PET/CT as prognostic biomarker and predictor of treatment response and
its role in detection of tumour recurrence after treatment (follow-up).
1.2 [
18
F]FDG PET/CT
1.2.1 PET/CT
The unstable nucleus of the isotope bound to the radiopharmaceutical, ′uorine-18čin
the case of [
18
F]FDG, emits a positron (ß
+
) when it decays. This positron will slow
down in the tissue around its origin and will interact with any of the electrons (e
−
)
in the tissue within a few millimetres from its origin. As positrons are the anti-
particles of electrons, collision of positrons and electrons will result in disappear-
ance of both particles, so-called annihilation, releasing energy in the form of two
(nearly) anti-parallel 511 keV photons. By detecting these coincident, paired pho-
tons with a PET-camera, a line can be derived on where the annihilation took place,
a so-called line-of-response (LOR). When enough LORs are measured, the origin of
the annihilation can be retraced using a reconstruction algorithm, resulting in the
spatial distribution of the radiopharmaceutical, i.e. a PET-image (Fig. 1.1). Most
current PET/CT scanners have time-of- ight capabilities that can register the arrival
time of each of both photons on a detector separately, allowing further localisation
(with a certain probability) of the point of annihilation on the LOR. Current clinical
scanners have time-of-′ight coincidence timing resolutions below 210čps, translat-
ing to a position uncertainty of less than 31.5 mm [6].
The magnitude of the signal depends on the number of degradations observed by
the camera. This in turn is dependent on the concentration of the radiopharmaceuti-
cal, thus the pharmacokinetics of the injected radiopharmaceutical and the probabil-
ity that a photon-pair is detected by the PET camera. The latter is, apart from
detector sensitivity and scanner geometry, mainly limited by photon absorption
(attenuation) in the tissue that both annihilation photons have to traverse before
reaching the detector ring, which can be measured by CT. Therefore, for the purpose
of attenuation correction and anatomical localisation, a low-dose CT is performed,
usually immediately prior to PET-acquisition.
In order to obtain images of acceptable image quality, suf≥cient LORs need to be
measured and, therefore, whole-body PET-acquisition is a time-consuming process,
typically in the order of 15-30 min, to keep the injected amount of radiopharmaceu-
tical, and thus the effective radiation dose for the patient, within reasonable limits.
1.2.2 [
18
F]FDG
The most frequently used radiopharmaceutical for PET-imaging is [
18
F]FDG [7].
The isotope
18
F mainly (96.9%) decays by ß
+
-emission, with a decay half-life of
109.7 min and a maximum (mean) positron energy of 634 (250) keV. The resulting
maximum (mean) positron range in water is 2.4 (0.6) mm before the annihilation,
localised by PET/CT, takes place [8].
H. E. E. Pouw et al.

7
Fig
. 1.1

The formation of a PET-image. (
a
) Electron–positron annihilation in the body within a few mm from the location of the radiopharmaceutical, resulting
in two nearly antiparallel 511 keV photons. (
b
) Coincidence detection of the paired annihilation photons by the PET detector. (
c
) Example of an attenuation and
scatter corrected PET-image, visualising a non-small cell lung carcinoma
1 Use of [
18
F]FDG PET/CT for Target Volume Denition in Radiotherapy

8
[
18
F]FDG is a radiopharmaceutical analogue to glucose. [
18
F]FDG is transported
by the blood stream, freely passes the vessel wall and extravascular extracellular
space similar to d-glucose, the natural occurring form of glucose. There it is actively
transported over the cellular membrane mainly by the sodium-dependent glucose
transporter molecule family and is subsequently phosphorylated to [
18
F]FDG-6-
phosphate by the hexokinase enzyme family in the cytosol. For both [
18
F]FDG and
d-glucose the kinetics of these processes are nearly identical. Glucose-6-phosphate
is catabolised to fructose-6-phosphate in the glycolysis pathway and eventually to
two molecules of pyruvate. However, the enzyme responsible for this chemical
reaction is not sensitive to [
18
F]FDG-6-phosphate. As a result, [
18
F]FDG-6-phosphate
cannot be degraded further and, as dephosphorylation does not occur in most mam-
malian tissues, accumulates in the cells (Fig. 1.2). Therefore, [
18
F]FDG uptake is
Fig. 1.2 The membrane-bound GLUT receptor family can reversibly transport both d-glucose
and [
18
F]FDG over the lipid bilayer of the cell. In the cytosol both d-glucose and [
18
F]FDG are
6-phosphorylated by the hexokinase isozymes. In contrast to d-glucose-6-phosphate, [
18
F]FDG-6-
phosphate is not a substrate for the subsequent isomerase enzyme in the glycolytic pathway and,
as 6-dephosphorylation hardly occurs in most mammalian tissues, its downstream products cannot
enter the TCA-cycle. As a result, [
18
F]FDG-6-phosphate will time-dependently accumulate in the
cytosol, re ecting enzymatic activities and facilitating PET visualisation of high glucose demand-
ing tissues. ATP adenosine-5′-triphosphate; CO
2 carbon dioxide; EES extravascular extracellular
space; [
18
F]FDG 2-[
18
F] uoro-2-deoxy-d-glucose; GLUT facilitative membrane-bound sodium-
independent glucose transporters; H
2O water; K
1–k
6 equilibrium Michaelis-Menten rate constants;
O
2 oxygen; TCA tricarboxylic acid (or Krebs) cycle. Adapted from ISBN13/EAN 978-94-6108-927-4
(ch.1, p.xxii) © 2015 Dennis Vriens. All Rights Reserved
H. E. E. Pouw et al.

9
increased in tissues with high glucose metabolism, often the case in brain and myo-
cardium, but also in cancerous lesions and in ammatory conditions.
Tumour cells acquire their adenosine-5′-triphosphate (ATP), the energy carrier
necessary for mitosis, from the inef≥cient anaerobic glycolysis, i.e. from pyruvate
to lactate instead of mitochondrial oxidative phosphorylation, i.e. from pyruvate to
carbon dioxide, even when oxygen is plentiful. The former yields 18–19x less ATP
than the latter. Due to this so-called Warburg effect, higher d-glucose metabolism is
required for suf≥cient ATP availability. This glucose demand is further increased by
the high cell proliferation [9].
1.3 Technical Aspects
To obtain high-quality images and to prevent large inter-investigation variability,
standardised patient preparation and image acquisition and reconstruction are of
uttermost importance. To ensure high-quality images and to overcome inter-investi-
gational variability in PET imaging, procedure guidelines are de≥ned by the
European Association of Nuclear Medicine (EANM) [10] and monitored for multi-
centre [
18
F]FDG PET/CT study purposes in the EANM Research Ltd. (EARL)
accreditation programme [11].
1.3.1 Patient Preparation
To ensure proper visualisation of the regions of interest, [
18
F]FDG should speci≥-
cally accumulate in the regions of interest. Uptake of [
18
F]FDG should be prevented
in non-malignant tissues, such as active muscles and brown fat, by proper patient
preparation. Patients should refrain from strenuous muscle activity before [
18
F]FDG-
injection and from any muscle activity after [
18
F]FDG-injection. To prevent uptake
in thermogenic brown-adipose tissue and muscle uptake by shivering, the resting
period after injection of the radiopharmaceutical takes place in a warm environ-
ment. A stable normoglycaemic, hypoinsulinaemic situation should be strived for
by injecting [
18
F]FDG only in patients who fasted for at least 4–6 h, are normogly-
caemic (generally a serum glucose up to 11.1 mmol/L (i.e. 200 mg/dl) is accepted)
and have not been injected with short-acting insulins for at least 4 h. Adequate
hydration and voiding directly prior to PET/CT acquisition ensure fast clearance of
the radiopharmaceutical and thus decrease dose to the patients, and prevent artefacts
around the kidneys, ureters and urinary bladder. When the area to be imaged is
located near the bladder, diuretic medication can be administered to further stimu-
late the emptying of the bladder during acquisition. To prevent metal artefacts, the
patients are asked to remove all metal objects [10].
1 Use of [
18
F]FDG PET/CT for′Target Volume De−nition in′Radiotherapy

10
1.3.2 Image Acquisition and Reconstruction
After acquisition of the raw PET-data, reconstruction can be performed by mainly
two methods, namely ltered backprojection (FBP) and iterative reconstruction. In
FBP, the LORs are literally backprojected to recalculate the position of the [
18
F]FDG-
containing structure. Currently, the most commonly applied reconstruction tech-
nique in the clinical setting is iterative reconstruction. This method outperforms
FBP by its high signal-to-noise ratio, the exclusion of counts outside the eld of
view and the multiple correction options within the reconstruction, e.g. time-of-
ight correction.
These advantages most often outweigh the computational intensiveness of the
reconstruction and the unpredictability of the nonlinear method. Iterative recon-
struction is an iterative process of adjusting an estimated image, and is based on the
difference between the estimated image of the projection data with the measured
projection data. The most well-known example of iterative reconstruction is the
ordered subsets expectation maximisation algorithm (OSEM), where the computa-
tional intensiveness is decreased by using only a subset of the projection angles in
every iteration. This process is repeated over all subsets to ulimately perform one
true iteration of OSEM [12]. A more extensive, but accessible, explanation of the
reconstruction methods can be found in Cherry et al. [12].
Before reconstructing the raw PET-data, corrections need to be applied for sev-
eral physical factors to obtain a true re ection of the radioactivity distribution of the
radiopharmaceutical. The most important corrections are variations in coincidence
detection efciency between the different LORs (normalisation), dead time of the
detector after a coincidence, random coincidence photons, scattered coincidence
photons, and absorbed (attenuated) coincidence photons. As explained before, a low
dose CT, acquired without contrast and often prior to PET-acquisition, is used for
the attenuation correction of these coincidences and is also called the attenuation
correction CT (CTAC). The amount of attenuation of 511 KeV annihilation photons
depends on the (electron) density of the tissue. The photons will be more attenuated
when passing through denser material, i.e. components with higher Hounseld
units, or when traversing more material, i.e. a larger body part [13].
1.3.3 Fusion and Registration of Treatment Planning PET/CT
1.3.3.1 Registration PET with CTAC
For correct anatomical localisation and attenuation- and scatter correction, the (low
dose) CTAC and the PET need to be spatially aligned. Spatial mismatch between the
PET and the CTAC results in incorrect attenuation correction and thereby scatter
correction. This incorrect correction causes over- or underestimation of activity
concentrations of the radiopharmaceutical (i.e. intensity), which is used to dene
the boundaries of the target volume on PET. Therefore, inaccurate attenuation- and
scatter correction, due to spatial mismatch, also leads to inadequate information for
target volume delineation.
H. E. E. Pouw et al.

11
Currently, all clinical PET/CT-scanners are dual-modality imaging devices,
where the hardware accurately aligns the images of PET and CT, provided that no
patient movement between the CTAC-acquisition and PET-acquisition has occurred.
When (a large amount of) patient movement has occurred, PET and CT images have
to be co-registered after acquisition, by translation, rotation and rescaling (rigid
transformations) or by image deformation (non-rigid or elastic transformations),
brie y summarised as ?image registration? [14].
1.3.3.2 Registration PET(/CT) with Planning-CT
Biological information from the PET-image can be used for treatment planning
by fusing either a staging PET or, preferably, a PET in treatment position with
the high-dose and often contrast-enhanced planning-CT. In case of spatial mis-
match between the PET and the planning-CT, improper functional-anatomical
co-localisation may occur, resulting in inaccurate target volume denition. This
can be overcome by manual co-registration focussing on the GTV and thus the
volume planned to receive the high radiation dose. If misalignment persists,
target volume coverage may be compromised, increasing the risk of geometrical
miss. Therefore, the registration needs to be veried by an experienced radiation
oncologist.
Imaging on different modalities increases the chance of inaccurate fusion by dif-
ferences in time frames, patient position and coordinate systems [15]. Acquisition
of both scans on the same machine and preferably at the same time would, there-
fore, minimise the spatial mismatch, but is often not realistic in clinical practice.
When the planning-CT is obtained on the PET-system, this PET/CT-system needs
to full the quality assurance for treatment planning and thus needs to be regularly
checked for its technical specications, such as table rigidity and levelness, accurate
laser alignment, and CT image matrix alignment [16]. Due to these additional, fre-
quent quality controls on the one hand and device capacity on the other, multiple
institutions use a stand-alone CT-scanner, instead of a hybrid PET/CT to perform a
planning-CT.
The PET/CT for clinical diagnosis and staging purposes (staging PET/CT) is
often one of the rst steps in the clinical work-up before the decision for treatment
modality has been made. Therefore, this staging PET/CT is not performed in radio-
therapy position. A staging PET/CT is usually performed on a soft curved table
top, while treatment planning scans are performed on a hard, rigid, at radiother-
apy table top, identical to the radiotherapy setting. The position of the patient dur-
ing a staging PET/CT is optimised for both patient comfort and image quality, e.g.
with a cushioned head rest and with the arms elevated, to prevent thorax imaging
artefacts by beam hardening and truncation. Patient-positioning during treatment
planning PET/CT is optimised for reproducibility and radiation-beam arrangement
avoiding dose in body parts not belonging to the target volume, e.g. arms in tho-
racic tumours. Therefore, rigid transformations cannot fully compensate for the
patient position differences. Software to deformably register staging PET/CTs
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12
with a planning-CT is currently commercially available and shows potential
improvement in alignment, even in case of preoperative PET/CT with a postopera-
tive CT [17–22]. The staging PET, however, can only be used for radiotherapy
planning when performed shortly before start of radiotherapy. This is a pre-requi-
site to prevent disease progression in the meantime, resulting in understaging and
undertreatment of the disease [23, 24].
1.3.4 Motion
Patient movement complicates accurate co-registration between the PET and the
CTAC, and between the PET(/CT) and the planning-CT, with the above-described
consequences. The effect on attenuation and thereby scatter correction, will espe-
cially be large in moving tumours at the interface of two types of tissue with large
differences in Hounseld units (photon attenuation), for example, the lower part of
the lung and the liver dome around the diaphragm [25]. Motion also negatively
in uences the PET-image, when occurring during the relatively lengthy PET-
acquisition itself, since it causes image-blurring, i.e. an overestimation of the vol-
ume of pathological radiopharmaceutical uptake and an underestimation of the
concentration of radiopharmaceutical.
1.3.4.1 Types of Motion
Motion can be divided into external patient motion (e.g. rotation of the head, open-
ing of the jaw) or internal target motion (e.g. bladder lling, diaphragm motion
during breathing, bowel motion).
External patient motion is mostly non-repetitive and effectuated by skeletal mus-
cles innervated by the somatic nervous system. It can be bulky, for example, when
the whole body or a full extremity is moved, which can be minimised using xation
devices and patient instruction. External patient motion can be more challenging in
patients who are not able to cooperate in minimising motion, e.g. very young
patients, patients with reduced mental capacities, who may require sedation.
External patient motion can also be compact such as uncontrollable tremor as a
result of, e.g. Parkinson’s disease.
Internal target motion is mostly involuntary, periodic or oscillatory in nature
and is caused by autonomic (mainly sympathetic) nerve innervation of effector
muscles, such as the myocardium and respiratory muscles, including the dia-
phragm. Aperiodic involuntary internal target motion, such as bowel motion, blad-
der lling or, at least partly, eye movement, should also be taken into account. Most
forms of internal target motion are unpreventable and require special measures to
deal with.
Methods to compensate for motion during the PET-acquisition will be high-
lighted here. Methods to compensate during the radiotherapy fraction itself are out-
lined elsewhere.
H. E. E. Pouw et al.

13
1.3.4.2 Motion Correction Methods
Alignment
Minimising spatial mismatch between scans and irradiation is achieved by per-
forming all image acquisitions and the treatment in highly similar (i.e. reproduc-
ible), position (preferably radiotherapy position), with respect to the internal
coordinate system (isocentre) of the scanners and treatment gantry. Accurate align-
ment can be achieved with the help of external lasers and skin (tattoo) markers. To
prevent deformations, immobilising devices can be helpful. These immobilising
devices can be personalised, for example, thermoplastics masks, bite blocks, and
some headrests, or generic, such as overlay beds, bands, strap restraints, sand bags,
shoulder depressors, head clamps, etc. Evaluation of the position directly before
the start of a radiotherapy fraction and directly afterwards can be achieved by
online imaging, where the 2D or 3D generated radiographic images, optional
including ducial markers, are compared with the planning-CT and its 2D digital
reconstructed radiographs [26].
Minimising patient motion during acquisition can largely be achieved by clear
patient instructions, starting with an understandable explanation of the importance of
motionlessness. Moreover, immobilising devices could improve the compliance.
Patients with postural pain despite adequate analgesia may be helped with reduced
acquisition time, which can be compensated for with higher injected dose of the
radiopharmaceutical to avoid deterioration of image quality (generally: halving
acquisition time requires doubling the radiopharmaceutical dose). Alternatively,
unconventional positioning can be attempted, but this will severely hamper the use of
these images for radiotherapy planning. When the target volume is located near the
bladder or urethra, for example, in rectum or cervix carcinoma, spatial mismatch can
arise between the PET and CT, due to bladder lling between both acquisitions. For
this purpose, imaging is generally performed directly after voiding and the CTAC is
acquired craniocaudally followed by the PET caudocranially, to keep the time inter-
val between PET and CT in the pelvic area as short as possible. In very anxious
patients, benzodiazepines can be prescribed or even sedatives can be administered.
Respiratory motion-induced interplay effects contribute most to intra-
acquisitional and intrafractional motion of breast, lung, liver and pancreatic tumours,
due to the period of normal breathing (2–5 s/breath) with respect to the duration of
PET-acquisition (2–5 min per bed position) or a therapeutic fraction (20–40 s per
beam angle) [27]. The contemporary methods to compensate for respiratory motion
will therefore be discussed separately in the next paragraph.
Compensation Methods for Respiratory Motion
Methods to control respiratory motion can be divided into four motion-encompassing
categories. The rst category exists of external compression methods, such as the
abdominal compression technique, where an external device applies pressure on the
abdomen, to minimise motion in the diaphragm. These methods are especially useful
for tumours close to the diaphragm, e.g. liver dome and lower lung lobes [28].
The second category contains controlled breathing methods, i.e. instead of free
breathing, instructions are provided to the patient to hold their breath on a specic
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moment in the respiratory cycle or to force shallow breathing. An automatic breath
control device, used in the active breathing control-technique, can also be used to
regulate the respiratory cycle.
By respiratory gating, being the third motion-encompassing category, the respi-
ratory cycle is measured. The longest period in the respiratory cycle where the least
thoracic volume changes occur is determined and the corresponding frames in the
list-mode acquired PET-data are used for reconstruction, resulting in a ‘frozen
image’. This gating method generally requires longer acquisition of thorax and
upper abdomen bed positions, since only a part of the acquired data is used for
image reconstruction, making it a lossy technique. Newer techniques are able to
only acquire PET-data at the usable moments in the respiratory cycle.
The last motion-encompassing method uses the PET-data itself for gating pur-
poses and, therefore, requires no additional hardware. Data-driven gating denes
an optimal binning strategy by principle component analysis. It encompasses the
elastic deformation of the images post-acquisition. With this technique all acquired
data are used during image reconstruction and it is, therefore, called a lossless tech-
nique [29]. Correction of lesion-specic motion seems possible by the positron
emission particle tracking technique combined with time of ight information, but
its clinical added value currently still needs to be determined [30].
One should keep in mind that the result of motion mitigation used for PET should
match that of either CTAC and treatment-CT to prevent spatial functional-anatomical
mismatch with all earlier mentioned disadvantages. Apart from the technical possi-
bilities, one should consider the clinical value of motion compensation. When no
motion compensation will be applied during the delivery of radiotherapy, motion
correction during PET/CT- and treatment planning-CT-acquisition might be unde-
sired. Therefore, some institutions use the ungated CT images for treatment plan-
ning. Using magnetic resonance-based linear accelerator for radiotherapy,
intrafractional adjustment of the treatment plan to motion is possible (Chaps.
6 and 7).
1.4 Target Volume Delineation
1.4.1 PET/CT in Target Volume Delineation
Adding biological PET/CT-information, sometimes addressed to as biological or
metabolic target volume, to the planning-CT could be helpful to clarify the nature
(benign versus malignant) of a tumour difcult to differentiate on solely CT. It is
assumed that in some tumour or scenarios tumour borders can be dened more
sharply, by more clearly differentiating malignant from non-malignant tissue (e.g.
post-obstructive atelectasis), by distinguishing ambiguous lymph nodes, and by dif-
ferentiating residual or recurrent disease from post-treatment scar tissue [31–33].
The addition of PET/CT in TVD has shown to lower interobserver variability in
several disease sites, compared to CT-based delineation [34–37]. The added value
of PET/CT target volume delineation depends on, e.g., the applied segmentation
H. E. E. Pouw et al.

15
ab
Fig. 1.3 (a) The target volume (GTV based on CT in yellow and CTV in red) in a non-small cell
lung cancer patient delineated on CT only, (b) the target volume delineated with the additional
information of [
18
F]FDG PET/CT (GTV based on PET in yellow, same CTV in red)
technique and tumour location and type, which will be discussed in the following
sections. In the remainder of this chapter, the PET(/CT)- and planning-CT-based
target volumes are referred to as PET-TV and CT-TV, respectively. A comparison
between a PET-TV and a CT-TV is provided in Fig. 1.3.
1.4.2 PE
1.4.2.1 M
Segmentation Methods
The contoured target volumes depend on the used segmentation method, which
classies the voxels of an image as either being malignant or non-malignant [38].
There are multiple segmentation methods under development, with different levels
of complexity and intuitiveness.
Manual tumour segmentation, still most common in clinical practice, is based on
visual inspection of the spatial [
18
F]FDG-distribution, combined with the available
anatomical information. This method requires interpretation, which makes it
extremely prone to observer variability. Standardised instructions are therefore
desired. Recommendations are given in head and neck squamous cell carcinoma
(HNSCC) by Lee et al. [39] and Schinagl et al. [40].
However, since radiation treatment planning is still based on anatomical
CT-imaging, PET ought to be seen as additional source of information on the GTV,
not as a replacement of CT.
As PET is a quantitative imaging modality, alternative (semi)automatic tumour
segmentation approaches have been developed which have shown to be more repro-
ducible than manual delineation. Those methods are described next.
Thresholding is, after manual delineation, the most applied and intuitive approach
in clinical practice. Multiple techniques have been proposed to determine the opti-
mal threshold intensity value of a quantitative PET-parameter to discern benign
from malignant voxels. The most often used semi-quantitative PET-parameter is the
standardised uptake value (SUV). This is the activity-concentration in a voxel (in
1 U
18
F]FDG PET/CT for Target Volume Denition in Radiotherapy

16
Bq/mL) normalised for administered activity of the radiopharmaceutical per unit
patient bodyweight (in Bq/g). This threshold can be set as an absolute threshold
(e.g. SUV ≥ 2.5 g/ml) or related to the mean, peak or maximum intensity in the
lesion (e.g. SUV ≥ 50%·SUV
max). Alternatively, an internal reference is used, such
as the lesion-to-background ratio. It is likely that the optimal threshold value
depends on tumour location (background, movement) and tumour size (with respect
to PET-scanner resolution). A clear outline of multiple threshold value formulas is
provided by Zaidi et al. [41].
A less intuitive category of segmentation methods, variational methods, is based
on spatial intensity variation between the foreground and the background. Examples
are edge and ridge detection methods, such as Sobel operators and Watershed trans-
formations, or active contouring methods. Drever et al. [42] provide a comprehen-
sive comparison between the Sober, Watershed and thresholding approaches for
PET TVD. An active contouring method, better known as a snake, is a spline, a
function de≥ned by multiple polynomial sub-functions, which incorporates prior
knowledge, e.g. smoothness and shape, to deform around the object. Active con-
touring makes subpixel contouring possible [43, 44]. These variational segmenta-
tion methods are complicated due to their sensitivity to image noise, especially in
case of gradient-based methods and when the lesion is surrounded by metabolic
active areas [15, 45, 46].
Segmentation can also be performed by a pattern recognition learning approach,
also known as machine learning. A distinction should be made between classi cation
in case of supervised learning on the one hand, i.e. when the nature (benign vs malig-
nant) of each voxel is known, and clustering by unsupervised learning on the other
hand. Both techniques are based on the extraction of features from the image.
Classi≥cation is widely applied in other imaging modalities, but may be hampered in
PET by the large heterogeneity in tumour uptake. Clustering, on the contrary, seems
valuable in target segmentation in PET. The most simple and common clustering
method is the k-means-algorithm, encompassing a ‘hard’ boundary that includes
every voxel in one of the two clusters, ‘tumour’ or ‘no tumour’. The centres of the
clusters are initialised and updated during the algorithm, until the ≥nal clustering is
retrieved. The boundaries can also be soft, allowing uncertain voxels to be probabi-
listic part of both clusters, which is synonymised as fuzzy. Examples of these soft
boundary clustering methods are fuzzy k-means or fuzzy C-means algorithms. These
computational complex learning methods, especially supervised learning, can pro-
vide much exibility, but can also be challenging and counterintuitive [39, 47].
Within the increasingly popular ≥eld of radiomics, a large number of imaging
features are extracted, which may contribute to the TVD, especially in learning
segmentation [48–50]. By using a deep learning approach, features can be extracted
automatically. Huang et al. [51] and Guo et al. [52] contoured the GTV of HNSCC
patients on PET/CT-images accurately and with high ef≥ciency with a deep neural
network, compared to more conventional segmentation methods. The gold standard
was considered manual contouring.
The last category of segmentation methods, statistical image segmentation,
differentiates between tumour uptake and surrounding tissue by probabilistic
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17
calculation and estimation of the data. This stochastic modelling approach deals
well with high noise PET-data, but is at risk of local optimal solutions in the
optimisation. An example is the Gaussian mixture model, which handles the
intensities as independent and Gaussian distributed. This dependency is not nec-
essary in the usage of hidden Markov models, another statistical segmentation
method [53]. Hatt et al. [54, 55] improved the segmentation of small and/or
heterogeneous lesions with the statistical segmentation algorithm fuzzy locally
adaptive Bayesian.
Shepherd et al. [56] performed a double-blind comparative study for 30 different
and combined segmentation techniques in 2012, from manual to full automatic and
concluded that up to then, manual segmentation outperformed all the (semi-)auto-
matic delineation methods in highest overall accuracy. An overview of the PET-
segmentation methods can be found in Zaidi et al. [41] and Hatt et al. [54].
1.4.2.2 Drawback of Segmentation Methods
Although manual segmentation is still most commonly performed in clinical prac-
tice, it shows a higher interobserver variability than the application of (semi-)
automatic segmentation methods [41]. The accuracy of manual segmentation
depends on the experience and expertise of the observer and proposes the risk of
observer overrating. Rasch et al. [57] showed that updating the PET-TV dened
by the treating physician with the assessment of 5–7 radiation oncologists and a
radiologist leads to an alteration in 45% of the cases. Manual delineation and
especially in agreement of multiple experts, preferably both radiation oncologists
and medical imaging specialists, is labour-intensive and hardly feasible in current
high demanding-high throughput oncology healthcare. Therefore, the develop-
ment of a reliable automatic PET segmentation method (PET-AS) is desired.
Automatic segmentation of PET-only data is currently under development,
although automatic PET segmentation with CT-data outperforms automatic PET-
only segmentation, with the disadvantage of increased complexity. For applica-
tion of PET-AS, the algorithms should be accurate and precise under different
clinical circumstances. Unfortunately, none of the currently existing algorithms
fulls all needs [48].
1.4.2.3 Consensus Algorithms
The success of multiple segmentation algorithms is strived to be combined by two
new methods: the Majority Vote and Simultaneous Truth And Performance Level
Estimate (STAPLE) method with their variances. The rst method decides whether
a voxel is included in the target volume, based on the outcome of the majority of the
methods. The latter estimates the segmentation result, based on a probability distri-
bution function of multiple separate segmentation methods and their performance
[58]. McGurk et al. [59] compared both consensus methods in ve (semi-) auto-
matic PET segmentation algorithms and concluded that both Majority Vote and
STAPLE were robust and more accurate for all separate segmentation methods in
different experimental circumstances. Schaefer et al. [60] and Dewalle-Vignion
et al. [61] conrmed these ndings, the latter for STAPLE in semi-clinical setting,
including comparison with manual segmentation.
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Berthon et al. [62] developed a third consensus method: the supervised machine
learning algorithm Automatic decision Tree-based Learning Algorithm for
Advanced Segmentation (ATLAAS), which includes a decision tree model to select
the most appropriate of 9 automatic segmentation methods, instead of combining
the results like the earlier described consensus methods. ATLAAS proved robust
segmentation with higher accuracy compared to each of the individual segmentation
methods in phantom setting and showed promising clinical results in HNSCC PET
tumour segmentation [62, 63].
For now, the International Atomic Energy Agency export report, published in
2009 and updated for non-small cell lung carcinoma (NSCLC) in 2019, recom-
mends that target volumes, delineated by a PET-AS technique, as part of standard
care, should always be visually checked by an experienced observer [64].
1.4.3 Disease Sites
1.4.3.1 NSCLC and SCLC
Several studies investigated the benecial effect of PET(/CT) on TVD. Most evi-
dence is available in non-small cell and small cell lung cancer (NSCLC and SCLC),
for which Hallqvist et al. [65] performed a systematic review of the role of PET/CT
in treatment planning. Their review shows a signicant change in PET-TV, com-
pared to CT-TV in approximately 40% of NSCLC patients. Moreover, in 20% of the
patients, the intent of the treatment was changed from radical high-dose to palliative
low-dose radiotherapy, as a result of upstaging of the tumour. In SCLC, target vol-
ume changed in 20% and treatment intention changed in 10% of the cases, when
PET is added to the TVD [65]. For both, NSCLC and SCLC, [
18
F]FDG PET guides
the primary tumour contour, in particular in case of atelectasis, even though the
tumour boundaries are drawn on the planning-CT in lung window-width/window-
level setting. The PET-TV results are only applicable for target volumes dened on
PET/CT-images in radiotherapy position, as discussed in ‘Technical aspects’ and
independent of the presence of a staging PET/CT.
De Ruysscher et al. [66] compared PET-TVs with CT-TVs in mediastinal lymph
nodes in a treatment planning study and subsequently treated all included NSCLC
patients according to the PET-delineated treatment plan in a single-arm prospective
clinical trial. The results obtained using the 3D-technique employed at the time showed
a low isolated lymph node recurrence rate of 2.3%. Bradley et al. [32] published their
ndings of the Radiation Therapy Oncology Group study in NSCLC patients. They
showed a disagreement of 51% comparing nodal PET-TV with CT-TV, mostly caused
by the in- or exclusion of one or two nodal stations. Also in this study, all patients
received irradiation according to the PET/CT derived treatment plan. In 2% of the
patients, i.e. 1 out of 46 patients, failure occurred in [
18
F]FDG-negative lymph nodes,
not included in the PET-TV, which is stated in this study as quite low. In 2018, the
MAASTRO group investigated the validity of [
18
F]FDG PET-based selective nodal irra-
diation in the era of Intensity-modulated radiation therapy [67]. They, again, reported on
an isolated nodal failure rate of 2.3%. So, since elective nodal irradiation is associated
H. E. E. Pouw et al.

19
with lower achievable doses and higher side-effects, selective nodal irradiation is to be
regarded the standard, also in the era of high conformal irradiation techniques.
In SCLC, again the MAASTRO group published their ndings on 60 SCLC
patients with limited disease SCLC [68]. They reported on an isolated nodal failure
rate of 3%, mainly in the supraclavicular fossa. The selective nodal approach, based
on [
18
F]FDG PET/CT, was subsequently employed in the CONVERT trial, showing
no detrimental effects on regional control [69].
The systematic review and meta-analysis of Hallqvist et al. [65] on [
18
F]FDG PET
in NSCLC included no studies with level I evidence, but nonetheless concluded that
?PET/CT for dose planning improves target denition and patient selection? in
NSCLC patients. Only in 2020, the PET-PLAN results were published, underlining
the results of the meta-analysis as well as recommendations by European Committees
(see below). Nestle et al. [70] randomly assigned 205 patients with inoperable
advanced stage NSCLC to PET/CT-based target volume including elective nodal
irradiation versus PET-based treatment planning, including selective nodal irradia-
tion only. At a follow-up time of 29 months at median, the locoregional progression
rate of the [
18
F]FDG PET-based group was non-inferior, and in fact even lower than
that for the conventional target group anticipated in the protocol, and also the toxicity
was non-inferior. Thus, the recommendation by the European Organisation for
Research and Treatment of Cancer (EORTC) in 2017, that PET/CT is standard for
treatment planning in lung cancer holds true, as do the guidelines adopted by the
European SocieTy for Radiotherapy and Oncology Advisory Committee in Radiation
Oncology Practice (ESTRO) guidelines in 2018 [71], and the Joint EANM/SNMMI/
ESTRO practice recommendations for the use of 2-[
18
F]FDG PET/CT external beam
radiation treatment planning in lung cancer V1.0, which was published in March
2022 [72].
1.4.3.2 HNSCC
In HNSCC, various publications have shown the PET-TV to be smaller than those
derived by CT or even MRI, when compared to histopathological resection speci-
men [73–80]. However, since the tumours originate from the oral mucosa and may
exhibit supercial tumour spreading not visible on macroscopic imaging, the value
of a thorough clinical examination is still high. Therefore, treatment planning solely
based on [
18
F]FDG PET/CT is still not standard of care.
Although Delouya et al. [75] and Chatterjee et al. [81] did not show signicant
changes in nodal target volume comparing PET-TV with CT-TV, Ciernik et al. [78]
showed target volume changes up to approximately 20%, based on the comparison
in six patients. Bearing in mind the high incidence of false-positive readings due to
reactive lymph nodes in the head and neck region, though, the [
18
F]FDG PET nd-
ing should always carefully be compared to that of the anatomical imaging modal-
ity, albeit CT or MRI [82, 83]. Instead, dose de-escalation based on the [
18
F]FDG
PET and CT nding may be investigated, which is at present the objective of a phase
II clinical trial [84–86].
In HNSCC, several guidelines recommend the application of [
18
F]FDG PET in
TVD, including those of the National Comprehensive Cancer Network and the
International Atomic Energy Agency [87].
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F]FDG PET/CT for Target Volume Denition in Radiotherapy

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1.4.3.3 Oesophageal Cancer
In a prospective study in oesophageal cancer patients, Ng et al. [88] included
[
18
F]FDG-avid disease, not identied by CT, in the PET-TV in 76% of 38 cases.
This changed the intent of the treatment from curative to palliative in 24% of 57
cases. Apolle et al. [89] compared the GTV based on [
18
F]FDG PET with that
dened by ducial markers implanted at the proximal and distal side of the tumour.
They concluded that the marker locations corresponded reasonably well with meta-
bolic tumour edges (mean: 5.4 mm more distally). The delineation of the gross
nodal volume may substantially change using [
18
F]FDG PET, as illustrated by
Jimenez-Jimenez et al. [90]. The use of PET/CT in the staging and planning of
radio(chemo)therapy seems to improve local recurrence-free survival in oesopha-
geal cancer [91]. The value of [
18
F]FDG PET in oesophageal cancer has recently
been extensively reviewed [92, 93].
1.4.3.4 Gynaecological Tumours
In gynaecological oncology, the primary tumour, e.g. of the cervix, is visualised
under clinical examination as well as by MRI (mainly T2-weighted imaging). MRI
is used for primary staging as well as for brachytherapy planning [94, 95]. However,
the value of [
18
F]FDG PET for diagnosis and radiation treatment planning, mainly
in terms of dening the nodal TV, is becoming increasingly apparent [77, 78, 88,
90, 96–99].
1.4.3.5 Lymphoma
The use of [
18
F]FDG PET/CT for treatment response monitoring and target volume
delineation in both, Hodgkin’s lymphoma and non-Hodgkin’s lymphoma, is
unequivocally highly recommended, as described by the International Lymphoma
Radiation Oncology Group [77, 100–104].
1.4.3.6 Other Tumours
The use of [
18
F]FDG-PET/CT for TVD in case of primary brain tumours or prostate
cancer is not benecial. Tracers enabling visualisation and delineation of those solid
tumours are discussed in Chap. 2.
It should be kept in mind that the success of PET TVD for nodal contouring
depends on its sensitivity and specicity [66, 78, 97]. For regional lymph node
detection, a high specicity between 0.90 (NSCLC) and 0.97 (cervical cancer) is
typically observed, versus a moderate sensitivity between 0.66 (oesophageal can-
cer) and 0.84 (HNSCC) [83, 105–110].
1.5 Cons and Pitfalls
In the previous sections, the added value of PET in treatment planning is sum-
marised. There are, however, some considerations that should be kept in mind when
applying this technique.
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21
1.5.1 Limited Spatial Resolution and High Signal-to-Noise
Ratio of PET
When using PET-imaging for TVD, one must be aware of the technical short-
comings of PET in general, which is shortly discussed in section ‘Technical
aspects’. PET is restricted by a limited spatial resolution of around 3–5 mm
full-width-at-half-maximum. Due to this low spatial resolution compared to
anatomical modalities such as CT and MRI, small lesions will be underesti-
mated in activity concentration or might even missed (i.e. false-negative), due to
the resulting partial volume effects. The consequences of this effect should be
considered in all small, low contrast lesions (typically <2 cm) and depend,
amongst others, on the resolution of the PET/CT-scanner, the applied recon-
struction algorithm, the post-reconstruction lter and the shape, size, uptake and
motion of the lesion [111]. Additional post processing and/or resolution model-
ling reconstruction algorithms can be applied to correct for partial volume
effects up to a certain extent, although the risk of introducing new artefacts
should be considered [54].
PET/CT-images can have a low signal-to-noise ratio, dependent of patient- and
institution-specic factors. The signal-to-noise ratio is, amongst others, negatively
in uenced in case of limited radiopharmaceutical uptake, when the injected dose of
the radiopharmaceutical is not adjusted to acquisition time (e.g. lossy respiratory
gating) or in case of large patient attenuation (high body mass index). The latter
might cause lesions to go undetected or treatment planning to become less repro-
ducible. On the contrary, the signal-to-noise ratio is enhanced by improved system
geometry (longer bores, whole-body PET-scanners), advanced (digital) detectors
and improved reconstruction algorithms (time-of- ight, Bayesian methods, deep
learning methods, smaller voxel sizes) [112, 113].
1.5.2 Interpretation Errors Due to Limited Specificity (and
Sensitivity) of the Radiopharmaceutical [
18
F]FDG
Although regions with high [
18
F]FDG-uptake represent tissues with high glucose-
demand, the amount of uptake does not fully depend on the presence and the degree
of malignancy. High physiological uptake in healthy tissue or low uptake in tumour
tissue can be physiological (e.g. brain, liver, urinary tract) or occur by improper
patient preparation (hyperglycaemia, hyperinsulinemia, some drugs, brown adipose
tissue). False-positive lesions can be found on PET/CT-images due to in ux of
in ammatory cells (lymphocytes, macrophages), due to infection (e.g. pneumonia),
in ammatory conditions (e.g. sarcoidosis) or after invasive intervention (e.g. biopsy,
radiotherapy). Another frequent cause of false-positive lesions are benign lesions
such as incidentalomas in the thyroid or intestines, that are misinterpreted as metas-
tases or secondary primaries and often require additional (invasive) diagnostics,
potentially leading to unnecessary delay in denite treatment.
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The success of lesion detection using [
18
F]FDG depends on the metabolic prole
of the tumour histology as some subtypes are notoriously false-negative (e.g. low-
grade tumours such as most neuroendocrine tumours and low-risk prostate cancer
or those with much extracellular matrix such as mucinous adenocarcinomas) and its
location (i.e. in regions with physiological high background including brain, liver,
kidneys and bladder) are other inuencing factors. Therefore, the disease-specic
sensitivity and specicity of the radiopharmaceutical should be considered. The
additional value of PET in nodal delineation is presumably caused by the in- or
exclusion of nodal stations, which depends on the error rate. PET’s high sensitivity
is especially useful in the inclusion of nodal stations in the GTV.
Altogether, it can be stated that lesions for treatment planning should ideally be
interpreted jointly by an experienced nuclear medicine physician and radiation
oncologist to place the images into context and to minimise the consequences of
false-positive or -negative ndings [66]. In case of doubt, it is recommended to
obtain cytological or histopathological conrmation. This is especially of impor-
tance when treatment would change from curative- to palliative intent, due to distant
metastases detected by PET.
1.6 Future Outlook
Radiation treatment outcome may be improved by incorporating patient-specic
tumour control probability and normal tissue complication probability in the treatment
plan, based on [
18
F]FDG-derived biological information of the tumour and the OAR.
[
18
F]FDG uptake in lesions, to some extent, re ects sensitivity of the tumour to
treatment. Less radiosensitive areas may prot from treatment intensication, e.g.
hypoxia modiers, immunotherapy, or radiation dose escalation. Personalised dose
escalation may be benecial in selected patients (stratication) and/or tumour sub-
volumes (subvolume boosting, dose painting by contours), even on voxel level
(dose painting by numbers).
To study feasibility, toxicity and efcacy of dose escalation in stage III
NSCLC patients, the PET-boost trial was initiated in 2010. In this multicentre
randomised phase II clinical trial, hypofractionated dose escalation was pro-
spectively studied in both the entire primary tumour (arm A) as in subvolumes
of the GTV, dened by increased [
18
F]FDG uptake (arm B). First, a planning
study reported by van Elmpt et al. [114] demonstrated feasibility of dose escala-
tion, both in the entire primary tumour (arm A) as well as in subvolumes of the
GTV (arm B). Subsequently, toxicity was tested. Preliminary results are reported
by van Diessen et al. [115]. Although increased acute and long-term toxicities
were observed in the study, the dose limits of the OAR were maintained. Efcacy
results were presented on the World Conference on Lung Cancer 2020 (which
took place virtually in January 2021). An excellent local control rate was
observed in both hypofractionated dose escalation arms, with a 2-year local
failure rate of less than 20% and a regional failure of only 27% [116]. Final
results are awaited.
H. E. E. Pouw et al.

23
Additional research is required into mechanisms of radioresistance, meth-
ods (radiopharmaceuticals, parameters) to adequately quantify radiosensitivity
and on techniques to accurately deliver the biologically adapted dose to the
patient. The relation between the imaging parameter and dose escalation should
also be studied, as well as the necessity of dose escalation (prescription func-
tion) [117].
Pre-treatment [
18
F]FDG PET/CT also provides plenty of information on the
surrounding tissue. This information might be useful in predicting toxicity in
the surrounding OAR, although limited studies are available yet. Van Dijk et al.
[118] showed that pre-treatment high metabolic parotid gland activity is associ-
ated with lower risk of developing late xerostomia. Anthony et al. [119] demon-
strated that pre-treatment [
18
F]FDG uptake in combination with CT lung texture
features in low-, medium-, and high-dose regions, could predict the risk of radi-
ation pneumonitis in lung cancer patients. Zschaeck et al. showed that [
18
F]FDG
PET uptake in normal tissue within irradiated HNSCC [120] and oesophageal
cancer [121] during treatment is a prognostic factor for local tumour control.
Besides these promising studies, more research should be performed to analyse
whether [
18
F]FDG PET image biomarkers (i.e. radiomics) of non-tumorous tis-
sue have predictive potential. The result of van Dijk et al. [118] should be veri-
ed in an independent dataset. To verify the ndings of Anthony et al. [118], a
larger patient population, in varying circumstances, with more positive cases is
required.
When the biological information provided by PET/CT of both the tumour and
the non-tumorous tissue is integrated in the treatment plan, this may result in a more
personalised treatment plan with maximal tumour control probability and minimal
normal tissue complication probability.
Acknowledgements We would like to thank dr. C.S. van der Vos for her valuable input, Mr.
G. Kracht for the design of Fig. 1.1 and prof. dr. E.G.C. Troost for writing the paragraph on the
clinical value of [
18
F]FDG PET for various tumour sites and for providing Fig. 1.3.
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31
2
Specific PET Tracers for Solid Tumors
and for Definition of the Biological
Target Volume
Constantin Lapa, Ken Herrmann, and Esther G. C. Troost
2.1 Introduction
As highlighted in Chap. 1, positron emission tomography (PET) is a key component
of primary disease staging in numerous solid tumors. For this means, mainly
2-[
18
F] uorodeoxyglucose-(FDG) is used, and [
18
F]FDG-PET-scans are in general
C. Lapa
Nuclear Medicine, Medical Faculty, University of Augsburg, Augsburg, Germany
e-mail: [email protected]
K. Herrmann
Department of Nuclear Medicine, University of Duisburg-Essen, and German Cancer
Consortium (DKTK)-University Hospital Essen, Essen, Germany
e-mail: [email protected]
E. G. C. Troost (*)
Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University
Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany
OncoRay—National Center for Radiation Research in Oncology, Faculty of Medicine and
University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum
Dresden-Rossendorf, Dresden, Germany
Helmholtz-Zentrum Dresden—Rossendorf, Institute of Radiooncology—OncoRay,
Dresden, Germany
National Center for Tumor Diseases (NCT), Partner Site Dresden, Dresden, Germany:
German Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Medicine and
University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany;
Helmholtz Association / Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden,
Germany
German Cancer Consortium (DKTK), Partner Site Dresden, and German Cancer Research
Center (DKFZ), Heidelberg, Germany
e-mail: [email protected]
© Springer Nature Switzerland AG 2022
E. G. C. Troost (ed.), Image-Guided High-Precision Radiotherapy,
https://doi.org/10.1007/978-3-031-08601-4_2

32
fused with anatomical imaging modalities, such as computed tomography (CT) and
magnetic resonance imaging (MRI). In radiation oncology, the entire [
18
F]FDG-
positive tumor volume is most commonly included in the irradiated target volume,
not taking into account underlying intratumoral heterogeneity regarding radiation
sensitivity or resistance.
In 2000, Ling et al. [1] introduced the concept of the biological target volume
(BTV). They postulated that different imaging modalities may unravel the underly-
ing tumor microenvironment and enable?by including this information?the de-
nition of different tumor subvolumes believed to require different radiation doses or
other forms of treatment. Already at that time, PET was postulated to be one of the
key imaging modalities for dening the BTV. Thus, in this chapter, PET tracers
beyond [
18
F]FDG, which depict (1) specic metabolic pathways of the primary
solid tumors, e.g., in primary brain tumors or prostate adenocarcinoma, and/or (2)
characteristics of tumor subvolumes, in head and neck squamous cell carcinoma
and non-small cell lung cancer will be presented.
2.2 Brain Tumors
Anatomical MRI, with its high soft-tissue contrast and spatial resolution, is the
cornerstone of delineating the tumor extent in primary brain tumors. However, MRI
is incapable of accurately depicting the tumor boundaries due to the tumors? inltra-
tive behavior. [
18
F]FDG cannot be reasonably used due to the physiological glucose
consumption of the healthy brain [2].
The proliferation marker [
18
F]-3′-deoxy-3′- uorothymidine ([
18
F]-FLT), that
accumulates in cerebral gliomas, is related to the grade of malignancy and progno-
sis [3, 4]. It may thus constitute a suitable alternative to [
18
F]FDG. However, it is
unable to identify the full extent of a glioma, because it is not capable of passing the
intact blood–brain barrier (BBB) and thus usually accumulates in tumor parts that
show disruption of the BBB, i.e., higher grade tumor subvolumes, as indicated by
contrast enhancement on MRI after application of paramagnetic contrast media [5].
In contrast to [
18
F]FDG, the uptake of radiolabelled amino acids is low in normal
brain tissue, rendering them appealing in brain tumors since tumor detection by
radiolabelled amino acid PET tracers is feasible with a high tumor-to-background
contrast. The increased uptake of amino acids seems to be predominantly caused by
increased transport of large neutral amino acids through the plasma membrane of
glioma cells via the L-type amino acid transporter (LAT) system subtypes LAT1
and LAT2 [6–8]. In addition, common amino acid PET tracers pass through the
intact BBB, which enables the depiction of the tumor mass beyond contrast enhance-
ment on MRI [9].
The longest-established amino acid tracer for PET is [
11
C]-methyl-l-methionine
([
11
C]-MET), but [
11
C]-MET–PET is restricted to only few neuro-oncology centers
because of the short half-life of [
11
C] (20 min) that necessitates an on-site cyclotron.
Consequently, amino acids labeled with [
18
F] (with the logistic advantage of a half-
life of 109.8 min), such as O-(2-
18
F- uoroethyl)-l-tyrosine ([
18
F]-FET) and
C. Lapa et al.

33
3,4-dihydroxy-6-
18
F- uoro-l-phenylalanine ([
18
F]-FDOPA), have been developed
[10, 11]. Several studies investigating the role of pre-treatment [
18
F]-FET-PET
either after surgery or after postoperative chemo/radiotherapy in glioblastoma dem-
onstrated amino acid PET-derived BTV to be highly prognostic for outcome, pro-
viding a rationale for the incorporation of amino acid PET in radiotherapy planning
[12–16].
Beyond MRI-based morphologic gross tumor volume (GTV) delineation, the
BTV may be dened by radiotracer uptake on amino acid PET that identies tumor
beyond standard MRI [17, 18]. Several studies examining primary tumor material
on a histopathological level have shown that amino acid PET may be more sensitive
to detect the true tumor extent (Fig. 2.1) [19–24]. The BTV dened by [
18
F]FET-
PET has been shown to extend beyond the MRI-dened GTV in a considerable
number of cases [25, 26] and the spatial congruence of MRI and [
18
F]FET-PET for
the identication of glioma GTV has been poor [27]. In addition, the metabolic
information provided by PET may identify subregions of tumor at higher risk of
recurrence, which can be included in the radiation boost volume in order to improve
the therapeutic ratio of radiation treatment. In this setting, PET-guided radiation
dose escalation to metabolically active foci in newly diagnosed glioblastoma
patients could be demonstrated to be feasible and safe; however, overall survival
was not prolonged so far [28].
Small studies analyzing patterns of failure following conventional radiochemo-
therapy based on standard MRI-dened tumor volumes suggest that amino acid
PET?dened tumor volumes may yield a more appropriate radiation target volume
[29, 30]. In these investigations, a proportionate increase in marginal or non-central
tumor recurrences was noted when regions of PET abnormality were not adequately
Fig. 2.1 Example of increased amino acid PET-derived BTV as compared to contrast-enhancement
as detected by standard magnetic resonance imaging. In a patient with newly diagnosed glioblas-
toma, biologic tumor volume derived by [
18
F]-FET-PET/CT is considerably larger than the volume
with contrast enhancement on conventional MRI. Modied from [18]
2 Speci c PET Tracers for Solid Tumors and for De nition of the Biological Target?

34
covered by high-dose radiation. Recently, a prospective trial investigating the asso-
ciation of time to recurrence in glioblastoma with [
11
C]-MET uptake before postop-
erative radiochemotherapy demonstrated a negative prognostic value of increased
tracer accumulation [31]. Of note, regions of PET positivity were often not detected
by MRI and exploratory analysis suggested a spatial correlation of the glioma recur-
rence region with pre-radiochemotherapy PET tracer accumulation in the majority
of patients, hinting at an added value of metabolic imaging beyond anatomical
information.
The impact of PET-based radiation treatment planning in recurrent high-grade
glioma was evaluated in a prospective single-institution trial [32], which showed a
signicantly improved overall survival when the target volume was delineated on
both amino acid PET and CT/MRI compared with CT/MRI alone. In 2016, the PET/
RANO report concluded that—in the setting of relapsed glioma—biological target
volumes may be more accurately depicted using amino acid PET tracers and that
metabolically more active tumor sub-volumes may be amenable for dose-painting
[7]. In this light, the current statement by the same group regarding the contribution
of PET imaging to radiotherapy planning and monitoring in glioma patients con-
rmed that the most frequently used radiolabeled amino acids MET, FET, and
FDOPA may improve the delineation of radiotherapy target volumes beyond con-
ventional MRI and identify additional tumor parts that should be targeted by irradia-
tion [13].
Currently, a multicenter phase II trial (GLIAA, NOA-10, ARO2013/1) is testing
the hypothesis that [
18
F]FET-PET-based re-irradiation is superior to radiotherapy
only based on conventional MRI [33]. However, the inclusion of amino acid PET-
based tumor volumes in standard-dose radiation therapy and re-irradiation protocols
continue to demonstrate a predominance of in-eld tumor recurrences, highlighting
the need for more effective therapies [28]. Additionally, the impact of amino acid
PET in comparison with advanced MRI techniques has not been proven yet and
warrants further investigation.
After radiotherapy, amino acid PET has consistently proven a useful tool in
radiotherapy response assessment in glioma patients with changes in tumor metabo-
lism harboring prognostic value for both progression-free and overall survival [12,
14]. Of note, this technique is particularly valuable in the differentiation between
radiation-related injury and glioma progression with a diagnostic accuracy between
80 and 90% [34, 35].
2.2.1 Glioma Hypoxia and Activated Microglia
Tumor hypoxia is characterized by an oxygen concentration below critical O
2 levels
and triggers several molecular, biological, and clinical effects, making it a negative
prognostic marker in nearly all solid tumors, including GBM. Hypoxia is a very
important tumor characteristic when considering tumor aggressiveness, leading to
the overexpression of the hypoxia inducible factor (HIF)-1 alpha, which enhances a
number of genes related to proliferation, thus making the tumor more radioresistant
C. Lapa et al.

35
[36]. Identifying hypoxia within gliomas may highlight the areas, which could
potentially benet from a radiation dose escalation.
Several PET radiopharmaceuticals have been developed to target hypoxia,
including the nitroimidazole-based compounds [
18
F]- uoromisonidazole
([
18
F]FMISO) and [
18
F]- uoroazomycin arabinoside ([
18
F]FAZA). [
18
F]FMISO and
[
18
F]FAZA are tracers that penetrate the cell by passive diffusion, and under low
oxygen concentration (pO
2 < 10 mmHg), are progressively reduced, leading to the
production of reactive radicals that bind covalently and irreversibly, to intracellular
molecules. Thus, both vectors accumulate in severely hypoxic tumor cells, which
are reported to be found mainly in high-grade glioma [37, 38]. Hypoxia PET has
been used pre-clinically to guide radiation treatment in a rat glioblastoma model
[39]. However, only a few clinical experiences are available on patients with high-
grade glioma undergoing hypoxia PET scans before and after radiotherapy [40, 41],
and this interesting approach has not achieved clinical relevance in brain tumor
diagnosis or treatment planning yet.
Another promising target for brain tumor imaging is the mitochondrial transloca-
tor protein (TSPO) [42]. Accumulation of TSPO ligands might extend beyond the
tumor margins on amino acid PET and indicate an inltration zone with activated
microglia as a marker of further tumor spread [43]. Further research to demonstrate
the suitability of this approach for radiotherapy planning is warranted.
2.2.2 Meningioma
Meningioma constitutes the most common primary brain tumor, with about 80%
being classied as WHO grade I, whereas WHO grade II and III meningioma are
less common [44]. If indicated, microsurgical resection is generally the therapy of
choice. Radiotherapy, including radiosurgery, which is predominantly used in the
recurrent situation, may be preferred in small WHO grade I meningioma or in loca-
tions not eligible for complete neurosurgical resection. Standard MRI is the imaging
method of choice and usually shows a homogeneous contrast enhancement and a
characteristic attachment to the dura mater [45].
On the molecular level, meningioma may express specic somatostatin receptors
(SSTR) [46]. SSTR expression can be non-invasively visualized using radiolabeled
SSTR ligands, usually compounds containing SSTR agonists, such as tyrosine3-
octreotate (TATE) or the octapeptide octreotide (TOC) and a chelator, e.g., tetrax-
etane (DOTA), coupled to the short-lived radionuclide gallium ([
68
Ga]). Since their
development in the 1990s, SSTR agonists have been increasingly used in meningi-
oma imaging. The main indication for SSTR-PET is identifying meningioma tissue,
including the delineation of meningioma extent, especially in complex anatomical
regions, such as the skull base or the orbital region, and with a special focus on the
diagnosis of intraosseous inltration [47].
Exact tumor delineation in complex anatomical regions, such as the skull base, is
not only crucial for surgical considerations but also of crucial importance for radio-
therapy planning, as the MRI-based morphologic GTV delineation may be
2 Speci c PET Tracers for Solid Tumors and for De nition of the Biological Target?

36
Fig. 2.2 Improved meningioma delineation using somatostatin receptor-directed (SSTR) positron
emission tomography/computed tomography (PET/CT). Beyond physiological tracer uptake of the
pituitary gland, SSTR-PET/CT facilitates tumor contouring in radiotherapy planning. Adapted
from [48]
insufcient to truly address the entire tumor volume. Particularly for the detection
of intra-osseous meningioma inltration or for the tumor delineation at the skull
base, PET using SSTR ligands has been shown to strongly complement anatomical
information from MRI and CT [47–49]. Inclusion of PET imaging for stereotactic
radiation treatment planning was not only able to identify inltrated tissue and pro-
vide information beyond bone windowing on CT and contrast enhancement on
MRI, but also has led to a preservation of critical areas, such as the pituitary gland
and the optic chiasm [50, 51].
Subsequent studies conrmed that an improved target volume delineation for
fractionated radiation therapy in patients with benign, atypical, and even anaplastic
meningiomas (WHO grades I–III) can be obtained by DOTATOC PET (Fig. 2.2)
[47, 48, 50].
Following treatment, SSTR-directed PET is reliably able to differentiate between
scar tissue and vital remnants or tumor recurrence [52].
2.3 Head and Neck Squamous Cell Carcinomas (HNSCC)
In recent years, signicant improvements in radio(chemo)therapy of head and neck
squamous cell carcinomas (HNSCC) have been achieved. In part, this is due to
improved imaging for accurate tumor staging and radiation treatment planning.
Apart from [
18
F]FDG-PET, eluded to in Chap. 1, PET tracers targeting the distinct
tumor microenvironment have been studied in prospective imaging trials regarding
their potential predictive and/or prognostic potential. These tracers include those
C. Lapa et al.

37
depicting tumor cell hypoxia, tumor cell proliferation, and other distinct tumor
characteristics.
Acute and chronic hypoxia originating from acute occlusion of blood vessels and
from insufcient diffusion of oxygen from the vessel to the tumor cell, respectively,
are known tumor characteristics adversely affecting outcome after surgery, chemo-
therapy, and also after radiotherapy [53]. These conclusions have been drawn from
studies including invasive measurement of the oxygenation status, e.g., using an
Eppendorff electrode, or from immunohistochemical staining of tumor biopsies
taken from HNSCC accessible for this invasive procedure [54–58]. However, both
this invasive nature of the assessment as well as the geometrical limitations of
immunohistochemical staining of tumor sections hampered the repeat use of this
procedure. PET imaging is capable of depicting tumor characteristics of the entire
tumor and owing to its non-invasive nature it can also be performed at several time-
points prior to and during treatment. Finally, a review on hypoxia imaging sup-
ported the correlation with treatment outcome [59].
The PET-tracers most widely used to image tumor cell hypoxia in HNSCC are
[
18
F]FMISO, [
18
F]FAZA, [
18
F] uoro ortanidazole ([
18
F]HX4), [
18
F] uorerythroni-
troimidazole ([
18
F]FETNIM), and diacetylbis(4-methylthiosemicarbazonato)coppe-
rII ([
62
Cu]Cu-ATSM), of which the rst has been most widely studied. The
underlying idea was and is that those tracers can be incorporated in dening the
biological target volume, ideally in the primary tumor and lymph nodes, and to
identify changes over time indicative of tumor changes requiring adaptation of the
treatment plan or even the form of treatment.
For [
18
F]FMISO, the entire chain from pre-clinical imaging of xenograft tumors
to inclusion in clinical studies and modeling efforts had been followed by a few
dedicated research sites. In Nijmegen, [
18
F]FMISO imaging of xenograft HNSCC
tumor models was correlated with levels of hypoxia identied by immunohisto-
chemical staining of the nitroimidazole pimonidazol on tumor sections. Overall, the
crude level of hypoxia correlated between micro- and macroscopic imaging, how-
ever, not on an individual xenograft tumor basis [60]. Also the signal of autoradiog-
raphy was found to correlate with levels of hypoxia under ambient conditions, after
carbogen breathing, and tumor clamping, again in three different xenograft tumor
lines [61].
In a modeling study on HNSCC and non-small cell lung cancer (NSCLC),
Eschmann et al. [62] were the rst to assess the prognostic value of [
18
F]FMISO
imaging prior to radiochemotherapy on both dynamic and static scans. A tracer
kinetic characterized by accumulation as well as high SUV 4 h after injection were
correlated with poor response to treatment. Rened kinetic models as well as a cor-
relation of [
18
F]FMISO with [
18
F]FDG were subsequently performed by the same
group [63–65].
Since then, several prospective clinical trials have assessed the value of (repeat)
[
18
F]FMISO-PET-imaging for patient stratication. The largest study to date was
initiated in 2005, performing [
18
F]FMISO-PET not only prior to treatment, but also
at several time-points during treatment, i.e., weeks 1, 2, and 5, as well as [
18
F]FDG-
PET scanning prior to, after completion and during follow-up visits. The study
2 Speci c PET Tracers for Solid Tumors and for De nition of the Biological Target?

38
consisted of a planned exploratory and validation cohort, each including 25 patients
with advanced-stage HNSCC of varying origin. In both parts of the study, the
hypoxia status, determined on the [
18
F]FMISO-PET of the second week of treat-
ment was found to be highly predictive for locoregional control [66, 67] (Fig. 2.3).
Recently, a prospective clinical phase II study in oropharyngeal cancer patients only
suggests that the radiation dose may be deescalated from the conventional dose of
70 to 30 Gy in tumors exhibiting no hypoxia on [
18
F]FMISO-PET prior to or with a
re-oxygenating tumor during radiation treatment [68].
Thorwarth et al. [69] were the rst to incorporate [
18
F]FMISO and [
18
F]FDG
information into a treatment planning study in 13 HNSCC. They found dose paint-
ing my numbers to deliver the radiation dose more effectively than an additional
uniform boost to [
18
F]FDG-positive areas. Based on the ndings of the [
18
F]FMISO
studies, both test and validation, performed in Dresden, Zschaeck et al. [70] came to
the conclusion that dose painting by numbers on [
18
F]FMISO-positive tumor sub-
volumes prior to treatment is insufcient to represent the ever-changing distribution
of [
18
F]FMISO, which became apparent by repeat [
18
F]FMISO imaging throughout
the course of treatment. The intra-tumoral stability of [
18
F]FMISO update was
insufcient for such an approach, and moreover, the site of local failure was in many
patients not within the [
18
F]FMISO-positive areas. Thus, the authors supported the
idea of utilizing [
18
F]FMISO as surrogate of tumor hypoxia per se and to rather
increase the radiation dose to the entire tumor, an approach, which is included in the
INDIRA-MISO (in preparation). This study design is also pursued in the phase III
FDG-PET/CT
baseline
FMISO-PET
baseline
FMISO-PET
week 2
Reoxygenation

Persisting hypoxia
FDG-PET/CT
during follow up
Local contro
l
Local recurrenc
e
A:
B:
Fig. 2.3 FDG-PET/CT at baseline and FMISO-PET at baseline and after week two of treatment
for two patients with tumors of similar size and location at the base of the tongue. While patient A
showed substantial reoxygenation after 2 weeks of treatment and was locally controlled despite
worse initial tumor stage (cT4 cN1), patient B (cT3 cN0) showed persisting hypoxia and an early
local recurrence developed. Reprinted with permission from [66]
C. Lapa et al.

39
randomized ESCALOX trial, supported by the German Research Council (DFG),
which tests the hypothesis whether radiation dose escalation to the GTV improves
2-year locoregional control and overall survival after concurrent radiochemotherapy
in HNSCC patients [71]. Radiation will be delivered to a total dose of 80.5 Gy using
a simultaneous integrated boost and cisplatin is administered either weekly or three-
weekly. In the translational part of this study, 100 patients will undergo [
18
F]FMISO-
PET twice in the week before treatment start in order to assess the presence of and
possible changes in tumor hypoxia.
The metabolic target volume (MTV) and hypoxic volume (HV) were assessed
based on [
18
F]FDG-PET and [
18
F]FMISO-PET imaging in 20 primary HNSCC
tumors and 19 metastatic lymph nodes [72]. When considering the intra-tumor
MTV of the [
18
F]FDG-PET and [
18
F]FMISO-PET, only 26% of the primary tumors
and 15% of the lymph nodes were found to strongly correlate. For the HV, only
19% of the primary tumors and 12% of the lymph nodes were strongly correlated.
On a quantitative level, correlations between both tracers were present for the pri-
mary tumors, but not for the lymph node metastases. Since high levels of both
[
18
F]FDG-PET and [
18
F]FMISO-PET can be found in selected tumors only,
[
18
F]FDG-PET is no surrogate to identify or predict intra-tumor hypoxia, the
authors concluded.
In the context of the EORTC 1219 clinical trial, the value of [
18
F]FAZA-PET is
being assessed in patients treated with radiochemotherapy and randomized to also
receive the hypoxia-modifying agent nimorazole, to evaluate [
18
F]FAZA-PET/CT
as a prognostic factor of the loco-regional control rate at 2 years in HNSCC patients
receiving radiochemotherapy ± nimorazole. Following their initial study on
[
18
F]FAZA-PET, Imaizumi et al. [73] published a comparative analysis on
[
18
F]FAZA-PET and diffusion-weighted magnetic resonance imaging (DWI-MRI).
In 11 patients investigated, the authors reported on a signicant positive correlation
between [
18
F]FAZA-PET and the slow compartment of a two/compartment model
for DWI, whereas diffusional kurtosis had a signicant negative correlation.
As part of the TROG 02.02 phase III clinical study, 41 HNSCC patients were
imaged using [
18
F]FDG-PET and [
18
F]FAZA-PET, both of which were assessed
qualitatively and quantitatively [74]. Using multivariate analysis, the hypoxic vol-
ume derived from [
18
F]FAZA-PET was found to signicantly correlate with the
T-stage, not with HPV-status or other adverse characteristics. Moreover, hypoxic
tumors treated with cisplatin had a signicantly worse treatment outcome relative to
oxic tumors as dened by [
18
F]FAZA-PET or hypoxic tumors treated with
tirapazamine.
Interestingly, a multicenter individual patient database meta-analysis of 163
hypoxia PET-scans ([
18
F]FMISO, N = 102; [
18
F]FAZA, N = 51) correlated PET
readings with locoregional control and overall survival [75]. Even though the base-
line characteristics signicantly differed between the cohorts, the commonly used
hypoxic parameters, i.e., maximum tumor-to-background radio (TMR
max) and
hypoxic volume with a 1.6 threshold (HV1.6) strongly correlated with locoregional
control and overall survival. It was thus concluded that both tracers appeared robust
and seemed equivalent in multicenter trials.
2 Speci c PET Tracers for Solid Tumors and for De nition of the Biological Target?

40
[
18
F]HX4-PET has been studied to a lesser extent in HNSCC patients. First, the
reproducibility and spatial stability of the marker was assessed in HNSCC [76].
Subsequently, in 2016, Zegers et al. [77] assessed changes of the tracer as well as of
blood biomarkers in 20 HNSCC patients. Within the GTV, the hypoxic fraction and
hypoxic volume were found to signicantly decrease in 69% of the analyzed GTVs
(N = 32). Conversely, the levels of the blood biomarkers, i.e., carbonic anhydrase IX
(CAIX), osteopontin and vascular endothelial growth factor (VEGF), did not change
or increased (osteopontin). A recent publication by the same group, rst author
Sanduleanu et al. [78], summarized results on the prognostic value of repeat
[
18
F]HX4-PET in 34 HNSCC patients included in two prospective clinical trials.
Patients were scanned prior to radio(chemo)therapy (N = 33) as well as during treat-
ment (N = 28). Noteworthy, the interval between the start of treatment and the per-
treatment scan varied from 3 to 17 days with a median of 13 days. Based on
[
18
F]HX4-PET, the hypoxic fraction and hypoxic volume were analyzed and corre-
lated with treatment outcome. The static [
18
F]HX4-PET images were not of prog-
nostic value, whereas the dynamic changes revealed a signicantly shorter local
progression-free survival and overall survival (OS) in patients with an increase in
the hypoxic volume and also a shorter OS in patients with residual hypoxia on the
per-treatment scan. Thus, these ndings are in line with reports on [
18
F]FMISO
ndings.
For completion, also the two hypoxia-related PET-tracers [
18
F]FETNIM and
[
62
Cu]Cu-ATSM should also be eluded to here, even though it has only been studied
in a limited number of publications [79–83]. In the early 2000s, Lehtiö et al. [81, 82]
performed initial assessments of [
18
F]FETNIM-PET and established analytical
methods. In 2019, 32 patients with locoregionally advanced HNSCC undergoing
concurrent radiochemotherapy underwent [
18
F]FETNIM-PET imaging prior to and
per-treatment after 5 weeks of treatment [83]. On multivariate analysis, maximum
SUV (SUV
max) of the primary tumor prior to treatment was correlated with worse
local control, whereas a high mid-treatment SUV
max was associated with worse
distant-metastasis free survival and overall survival. On multivariate analysis, tumor
grade and mid-SUV
max were signicant predictors of worse overall survival.
Besides hypoxia imaging, [
18
F]FLT-PET as imaging biomarker of tumor cell pro-
liferation in HNSCC was of interest in the early 2010s. Troost et al. [84, 85] showed
in two comparative analyses between immunohistochemical staining of tumor and
lymph node resection specimen on the one hand, and [
18
F]FLT-PET on the other
hand, that Ki-67 and IdUrd staining correlated well with [
18
F]FLT-PET in 17 pri-
mary tumors, but that [
18
F]FLT-PET was not capable of distinguishing metastatic
from reactive lymph nodes in 10 HNSCC patients. Subsequent studies by the same
group proved that [
18
F]FLT-PET holds high prognostic value regarding locoregional
control [85–87]. However, since the synthesis of the tracer is complicated, it has not
yet found its way into routine clinical practice, neither for oropharyngeal tumors,
esophageal carcinoma, nor for NSCLC [88, 89].
The broblast activation protein (FAP), which is highly expressed on the bro-
blasts of tumor stroma, is a relatively new biological target, which can be addressed
with suitable FAP inhibitors (FAPI) that can be labeled with several radionuclides
C. Lapa et al.

41
such as [
68
Ga] and [
18
F]. Syed et al. [90] have shown a high tumor-to-background-
ratio of the FAP-ligand along with signicant alteration of TV-delineation in
HNSCC patients. The value of PET using [
18
F]FAPI is being evaluated for a variety
of tumors in the context of a prospective register (NCT04571086). The value of this
novel radiotracer PET for radiotherapy planning is to be assessed in prospective
clinical studies with relevant oncological endpoints.
2.4 Non-small Cell Lung Cancer
[
18
F]FDG-PET/CT has been recognized as the key imaging method for staging of
(non-)small cell lung cancer [(N)SCLC], for radiation treatment planning, for
response monitoring during radio(chemo)therapy, and for detection of recurrent dis-
ease [91–94], see Chap. 1.
Similar to HNSCC, other PET tracers apart from [
18
F]FDG, which re ect tumor
characteristics, such as hypoxia, proliferation, or immune status, have been investi-
gated in (N)SCLC. This stems from the fact that glycolysis, depicted by [
18
F]FDG,
is not concordant to other parameters, e.g., hypoxia [95]. In this paragraph, there-
fore, the PET-tracers used in NSCLC will be brie y mentioned.
In terms of hypoxia, the same tracers reported for HNSCC have also been used
in NSCLC, i.e., [
18
F]FMISO, [
18
F]FAZA, or [
18
F]HX4. In a multicenter, phase II
clinical trial, [
18
F]FMISO-PET was used to determine the hypoxia status in NSCLC
patients [96, 97]. In this study, [
18
F]FMISO-positive patients received a boost of
70–84 Gy, depending on the dose to surrounding organs at risk, while the other
patients received the standard dose of 66 Gy. The overall and progression-free sur-
vival rates of the 54 patients included were 48.5 and 28.8%, respectively, at 3 years
after treatment. The median overall survival in [
18
F]FMISO-positive patients was
25.8 months, whereas it was not reached in the [
18
F]FMISO-negative patients.
Owing to the small number of patients, no dose-effect relationship could be estab-
lished in the [
18
F]FMISO-positive patient subset. So, also this phase II study under-
lines the fact that [
18
F]FMISO-uptake is strongly associated with poor prognosis
in NSCLC.
In a recent publication, hypoxia represented by both [
18
F]FMISO- and [
18
F]FAZA-
PET was compared to immunohistochemical analyses (GLUT-1, CAIX, LDH-5,
and HIF-1alpha) in 18 NSCLC patients undergoing primary tumor resection [98].
The SUV
max of [
18
F]FMISO was found to be higher than that of [
18
F]FAZA, but to
correlate well. Noteworthy, the results of PET-imaging were not correlated with
immunohistochemical results, regardless of the staining.
For [
18
F]HX4, the group from MAASTRO clinic performed subsequent studies
on the performance of the tracer, quantied the hypoxic status in NSCLC patients
prior to and during radio(chemo)therapy, and employed it in radiation treatment
planning studies [76, 99–101]. In 2013, Zegers et al. [101] dened the optimal
time-point of imaging [
18
F]HX4 statically, as being 4 h post injection. Subsequently,
the authors [100] compared [
18
F]HX4 with [
18
F]FDG-PET imaging in NSCLC
patients prior to radio(chemo)therapy. They found hypoxic tumor volumes to be
2 Speci c PET Tracers for Solid Tumors and for De nition of the Biological Target?

42
smaller than metabolically active volumes, with half of the tumors exhibiting a
good overlap between the two PET-tracers. In the other patients, there was a (par-
tial) mismatch. In a radiation treatment planning study, Even et al. [99] created
radiation treatment plans for 10 NSCLC patients. Dose escalation based on meta-
bolic subvolumes, hypoxic subvolumes, or on the entire tumor volume was found
to be feasible, with doses ranging as high as 107 ± 20 Gy for metabolic subvolumes
and 117 ± 15 Gy for hypoxic subvolumes. Since the PET-Boost trial, referred to in
Chap. 1, escalating the radiation dose on
18
F]FDG-PET subvolumes was still
recruiting patients and the tracer [
18
F]HX4 is not amply available, no dose escala-
tion trial has been initiated.
Finally, [
18
F]FLT-PET representing tumor cell proliferation has only been tested
in a few studies. It has been used to monitor treatment response during radiochemo-
therapy as well as during targeted therapy [102–104].
2.5 Prostate Cancer
Molecular imaging is widely used for detecting, staging, and restaging of prostate
cancer patients. While previously mainly FDG and cellular membrane lipogenesis
markers such as [
18
F]- and [
11
C]-Choline-PET contributed to the patient manage-
ment of prostate cancer patients this has nowadays almost completely shifted to
radiolabelled PSMA inhibitors. In contrast to the historically used metabolic PET
tracers, PSMA-PET visualizes the expression of the glutamate carboxypeptidase
PSMA (prostate-specic membrane antigen) representing a trans-membrane pro-
tein, which is highly expressed in the majority of prostate cancers [105]. Due to its
excellent performance PSMA-PET is currently implemented in the major clinical
guidelines for detection of biochemical recurrence, primary staging of high-risk
disease, in case of PSA persistence after primary treatment as well as for selecting
patients for PSMA radioligand therapy [106].
Another important aspect is the availability and accessibility of novel PET trac-
ers. Currently, FDA approvals for both [
68
Ga]Ga-PSMA and [
18
F]F-DCFPyL are in
place, whereas in Europe so far only [
18
F]F-PSMA 1007 has a local market authori-
zation in France. However, within the next months EMA approvals for both
[
68
Ga]Ga-PSMA, [
18
F]F-PSMA 1007 and potentially [
18
F]F-DCFPyL are expected.
To help navigate among the ever-increasing number of PSMA PET tracers, it is
important to state that the clinically best developed PSMA PET tracers diagnostic is
overall probably very similar justifying the overarching term of a “class of PSMA
PET tracers”. Major discriminators are routes of production and batch sizes (and
accordingly cost of goods) and pathways of excretion (predominantly renal vs. pre-
dominantly hepatic-biliary). Hepatic-biliary dominant excreted PSMA PET tracers
such as [
18
F]F-PSMA 1007 seem to have a certain advantage for local staging in the
prostate and prostate bed due to the lower activity in the surrounding urinary blad-
der; however, a higher rate of so-called unspecic bone uptake might negatively
affect the overall performance [107]. In summary, the novel class of PSMA PET
tracers plays an important role for the management of prostate cancer patients. In
C. Lapa et al.

43
the following part, the impact of the “class of PSMA PET tracers” on radiation
therapy planning will be discussed in more detail.
2.5.1 PET in Primary Staging
In the primary setting, PSMA-PET/CT imaging can be applied for initial staging in
patients with high-risk prostate cancer [108]. The recently published prospective
randomized phase III study proPSMA showed that PSMA-PET/CT favorably
impacts patient management since the accuracy for lymph node and bone metasta-
ses is higher as compared to conventional imaging changing subsequently also the
radiation treatment plan [109]. Moreover, PSMA-PET/CT signicantly reduced the
number of equivocal ndings at an overall lower radiation dose compared to con-
ventional imaging. Several additional retrospective analyses have also addressed
this issue. Dewes et al. [110] reported on a change in TNM stage in 8 of 15 patients
or modications of clinical target volumes (CTVs) and changes in prescribed radia-
tion dose in 5 and 12 patients, respectively. In another retrospective analysis,
PSMA-PET/CT led to major changes in the radiotherapy plan in approximately
one-third of the patients, especially when no elective radiation to the pelvic lym-
phatic drainage system was initially planned [111].
Recently, an academically driven prospective phase III trial randomizing patients
with unfavorable, intermediate, and high-risk proles to groups with or without
PSMA-PET prior to denitive radiotherapy planning (NCT04457245) has been ini-
tiated. The primary endpoint of this study is the progression-free survival after ini-
tiation of denite radiotherapy aiming for improvement in oncological outcome for
the PSMA PET arm.
Interestingly, a clear dose-response relationship was shown for prostate cancer
patients. The prospective multicenter phase III study “FLAME” reported that dose
escalation to intraprostatic MRI-derived tumor lesions resulted in a signicant
improvement in recurrence-free survival [112]. However, it can be assumed from
previous publications that the contouring of the intraprostatic tumor mass deter-
mined taking account the potential additional information of PSMA-PET might
lead to a further improvement [113–116]. Recently, Zamboglou et al. [117] reported
in 10 patients on the feasibility to escalate the tumor dose to 95 Gy while contouring
the intraprostatic tumor lesions based on [
68
Ga]Ga-PSMA PET. Based on this very
promising data, a prospective multicenter phase II study is currently investigating
focal dose escalation to intraprostatic tumor volumes derived by PSMA-PET/CT
and MRI (HypoFocal; DRKS00017570).
2.5.2 Salvage Radiotherapy in Recurrent Prostate Cancer
In patients with biochemical recurrence (BCR) salvage radiotherapy (SRT) repre-
sents the most important therapeutic option. Since the introduction of PSMA and its
implementation into all major clinical guidelines, BCR patients are offered—if
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—Non, mais je puis m'éloigner du foyer qui la produit.
—Elle est donc extérieure?
—Sans doute. Nous flottons dans un courant d'eau bouillante.
—Est-il possible? m'écriai-je.
—Regardez.»

Les panneaux s'ouvrirent, et je vis la mer entièrement blanche
autour du Nautilus. Une fumée de vapeurs sulfureuses se déroulait
au milieu des flots qui bouillonnaient comme l'eau d'une chaudière.
J'appuyai ma main sur une des vitres, mais la chaleur était telle que
je dus la retirer.
«Où sommes-nous? demandai-je.
—Près de l'île Santorin, monsieur le professeur, me répondit le
capitaine, et précisément dans ce canal qui sépare Néa-Kamenni de
Paléa Kamenni. J'ai voulu vous donner le curieux spectacle d'une
éruption sous-marine.
—Je croyais, dis-je, que la formation de ces îles nouvelles était
terminée.
—Rien n'est jamais terminé dans les parages volcaniques,
répondit le capitaine Nemo, et le globe y est toujours travaillé par les
feux souterrains. Déjà, en l'an dix-neuf de notre ère, suivant
Cassiodore et Pline, une île nouvelle, Théia la divine, apparut à la
place même où se sont récemment formés ces îlots. Puis, elle
s'abîma sous les flots, pour se remontrer en l'an soixante-neuf et
s'abîmer encore une fois. Depuis cette époque jusqu'à nos jours, le
travail plutonien fut suspendu. Mais, le 3 février 1866, un nouvel îlot,
qu'on nomma l'îlot de George, émergea au milieu des vapeurs
sulfureuses, près de Néa-Kamenni, et s'y souda, le 6 du même mois.
Sept jours après, le 13 février, l'îlot Aphroessa parut, laissant entre
Néa-Kamenni et lui un canal de dix mètres. J'étais dans ces mers
quand le phénomène se produisit, et j'ai pu en observer toutes les
phases. L'îlot Aphroessa, de forme arrondie, mesurait trois cents
pieds de diamètre sur trente pieds de hauteur. Il se composait de
laves noires et vitreuses, mêlées de fragments feldspathiques. Enfin,
le 10 mars, un îlot plus petit, appelé Réka, se montra près de Néa-
Kamenni, et depuis lors, ces trois îlots, soudés ensemble, ne forment
plus qu'une seule et même île.
—Et le canal où nous sommes en ce moment? demandai-je.

—Le voici, répondit le capitaine Nemo, en me montrant une carte
de l'Archipel. Vous voyez que j'y ai porté les nouveaux îlots.
—Mais ce canal se comblera un jour?
—C'est probable, monsieur Aronnax, car, depuis 1866, huit petits
îlots de lave ont surgi en face du port Saint-Nicolas de Paléa-
Kamenni. Il est donc évident que Néa et Paléa se réuniront dans un
temps rapproché. Si, au milieu du Pacifique, ce sont les infusoires
qui forment les continents, ici, ce sont les phénomènes éruptifs.
Voyez, monsieur, voyez le travail qui s'accomplit sous ces flots.»
Je revins vers la vitre. Le Nautilus ne marchait plus. La chaleur
devenait intolérable. De blanche qu'elle était, la mer se faisait rouge,
coloration due à la présence d'un sel de fer. Malgré l'hermétique
fermeture du salon, une odeur sulfureuse insupportable se
dégageait, et j'apercevais des flammes écarlates dont la vivacité
tuait l'éclat de l'électricité.
J'étais en nage, j'étouffais, j'allais cuire. Oui, en vérité, je me
sentais cuire!
«On ne peut rester plus longtemps dans cette eau bouillante, dis-
je au capitaine.
—Non, ce ne serait pas prudent,» répondit l'impassible Nemo.
Un ordre fut donné. Le Nautilus vira de bord et s'éloigna de cette
fournaise qu'il ne pouvait impunément braver. Un quart d'heure plus
tard, nous respirions à la surface des flots.
La pensée me vint alors que si Ned Land avait choisi ces parages
pour effectuer notre fuite, nous ne serions pas sortis vivants de cette
mer de feu.
Le lendemain, 16 février, nous quittions ce bassin qui, entre
Rhodes et Alexandrie, compte des profondeurs de trois mille mètres,
et le Nautilus, passant au large de Cerigo, abandonnait l'archipel
grec, après avoir doublé le cap Matapan.

CHAPITRE VII
LA MÉDITERRANÉE EN QUARANTE-HUIT HEURES.
La Méditerranée, la mer bleue par excellence, «la grande mer»
des Hébreux, la «mer» des Grecs, le «mare nostrum» des Romains,
bordée d'orangers, d'aloës, de cactus, de pins maritimes, embaumée
du parfum des myrtes, encadrée de rudes montagnes, saturée d'un
air pur et transparent, mais incessamment travaillée par les feux de
la terre, est un véritable champ de bataille où Neptune et Pluton se
disputent encore l'empire du monde. C'est là, sur ses rivages et sur
ses eaux, dit Michelet, que l'homme se retrempe dans l'un des plus
puissants climats du globe.
Mais si beau qu'il soit, je n'ai pu prendre qu'un aperçu rapide de
ce bassin, dont la superficie couvre deux millions de kilomètres
carrés. Les connaissances personnelles du capitaine Nemo me firent
même défaut, car l'énigmatique personnage ne parut pas une seule
fois pendant cette traversée à grande vitesse. J'estime à six cents
lieues environ le chemin que le Nautilus parcourut sous les flots de
cette mer, et ce voyage, il l'accomplit en deux fois vingt-quatre
heures. Partis le matin du 16 février des parages de la Grèce, le 18,
au soleil levant, nous avions franchi le détroit de Gibraltar.
Il fut évident pour moi que cette Méditerranée, resserrée au
milieu de ces terres qu'il voulait fuir, déplaisait au capitaine Nemo.
Ses flots et ses brises lui rapportaient trop de souvenirs, sinon trop
de regrets. Il n'avait plus ici cette liberté d'allures, cette
indépendance de manœuvres que lui laissaient les océans, et son
Nautilus se sentait à l'étroit entre ces rivages rapprochés de l'Afrique
et de l'Europe.

Aussi, notre vitesse fut-elle de vingt-cinq milles à l'heure, soit
douze lieues de quatre kilomètres. Il va sans dire que Ned Land, à
son grand ennui, dut renoncer à ses projets de fuite. Il ne pouvait se
servir du canot entraîné à raison de douze à treize mètres par
seconde. Quitter le Nautilus dans ces conditions, c'eût été sauter
d'un train marchant avec cette rapidité, manœuvre imprudente s'il
en fut. D'ailleurs, notre appareil ne remontait que la nuit à la surface
des flots, afin de renouveler sa provision d'air, et il se dirigeait
seulement suivant les indications de la boussole et les relèvements
du loch.
Je ne vis donc de l'intérieur de cette Méditerranée que ce que le
voyageur d'un express aperçoit du paysage qui fuit devant ses yeux,
c'est-à-dire les horizons lointains, et non les premiers plans qui
passent comme un éclair. Cependant, Conseil et moi, nous pûmes
observer quelques-uns de ces poissons méditerranéens, que la
puissance de leurs nageoires maintenait quelques instants dans les
eaux du Nautilus. Nous restions à l'affût devant les vitres du salon,
et nos notes me permettent de refaire en quelques mots l'ichtyologie
de cette mer.
Des divers poissons qui l'habitent, j'ai vu les uns, entrevu les
autres, sans parler de ceux que la vitesse du Nautilus déroba à mes
yeux. Qu'il me soit donc permis de les classer d'après cette
classification fantaisiste. Elle rendra mieux mes rapides observations.
Au milieu de la masse des eaux vivement éclairées par les
nappes électriques, serpentaient quelques-unes de ces lamproies
longues d'un mètre, qui sont communes à presque tous les climats.
Des oxyrhinques, sortes de raies, larges de cinq pieds, au ventre
blanc, au dos gris cendré et tacheté, se développaient comme de
vastes châles emportés par les courants. D'autres raies passaient si
vite que je ne pouvais reconnaître si elles méritaient ce nom d'aigles
qui leur fut donné par les Grecs, ou ces qualifications de rat, de
crapaud et de chauve-souris, dont les pêcheurs modernes les ont
affublées. Des squales-milandres, longs de douze pieds et
particulièrement redoutés des plongeurs, luttaient de rapidité entre

eux. Des renards marins, longs de huit pieds et doués d'une extrême
finesse d'odorat, apparaissaient comme de grandes ombres
bleuâtres. Des dorades, du genre spare, dont quelques-unes
mesuraient jusqu'à treize décimètres, se montraient dans leur
vêtement d'argent et d'azur entouré de bandelettes, qui tranchait
sur le ton sombre de leurs nageoires; poissons consacrés à Vénus, et
dont l'œil est enchâssé dans un sourcil d'or; espèce précieuse, amie
de toutes les eaux, douces ou salées, habitant les fleuves, les lacs et
les océans, vivant sous tous les climats, supportant toutes les
températures, et dont la race, qui remonte aux époques géologiques
de la terre, a conservé toute sa beauté des premiers jours. Des
esturgeons magnifiques, longs de neuf à dix mètres, animaux de
grande marche, heurtaient d'une queue puissante la vitre des
panneaux, montrant leur dos bleuâtre à petites taches brunes; ils
ressemblent aux squales dont ils n'égalent pas la force, et se
rencontrent dans toutes les mers; au printemps, ils aiment à
remonter les grands fleuves, à lutter contre les courants du Volga,
du Danube, du Pô, du Rhin, de la Loire, de l'Oder, et se nourrissent
de harengs, de maquereaux, de saumons et de gades; bien qu'ils
appartiennent à la classe des cartilagineux, ils sont délicats; on les
mange frais, séchés, marinés ou salés, et, autrefois, on les portait
triomphalement sur la table des Lucullus. Mais de ces divers
habitants de la Méditerranée, ceux que je pus observer le plus
utilement, lorsque le Nautilus se rapprochait de la surface,
appartenaient au soixante-troisième genre des poissons osseux.
C'étaient des scombres-thons, au dos bleu-noir, au ventre cuirassé
d'argent, et dont les rayons dorsaux jettent des lueurs d'or. Ils ont la
réputation de suivre la marche des navires dont ils recherchent
l'ombre fraîche sous les feux du ciel tropical, et ils ne la démentirent
pas en accompagnant le Nautilus comme ils accompagnèrent
autrefois les vaisseaux de Lapérouse. Pendant de longues heures, ils
luttèrent de vitesse avec notre appareil. Je ne pouvais me lasser
d'admirer ces animaux véritablement taillés pour la course, leur tête
petite, leur corps lisse et fusiforme qui chez quelques-uns dépassait
trois mètres, leurs pectorales douées d'une remarquable vigueur et
leurs caudales fourchues. Ils nageaient en triangle, comme certaines

troupes d'oiseaux dont ils égalaient la rapidité, ce qui faisait dire aux
anciens que la géométrie et la stratégie leur étaient familières. Et
cependant ils n'échappent point aux poursuites des Provençaux, qui
les estiment comme les estimaient les habitants de la Propontide et
de l'Italie, et c'est en aveugles, en étourdis, que ces précieux
animaux vont se jeter et périr par milliers dans les madragues
marseillaises.
Je citerai, pour mémoire seulement, ceux des poissons
méditerranéens que Conseil ou moi nous ne fîmes qu'entrevoir.
C'étaient des gymnotes-fierasfers blanchâtres qui passaient comme
d'insaisissables vapeurs, des murènes-congres, serpents de trois à
quatre mètres enjolivés de vert, de bleu et de jaune, des gades-
merlus, longs de trois pieds, dont le foie formait un morceau délicat,
des cœpoles-ténias qui flottaient comme de fines algues, des trygles
que les poëtes appellent poissons-lyres et les marins poissons-
siffleurs, et dont le museau est orné de deux lames triangulaires et
dentelées qui figurent l'instrument du vieil Homère, des trygles-
hirondelles, nageant avec la rapidité de l'oiseau dont ils ont pris le
nom, des holocentres-mérons, à tête rouge, dont la nageoire dorsale
est garnie de filaments, des aloses agrémentées de taches noires,
grises, brunes, bleues, jaunes, vertes, qui sont sensibles à la voix
argentine des clochettes, et de splendides turbots, ces faisans de la
mer, sortes de losanges à nageoires jaunâtres, pointillés de brun, et
dont le côté supérieur, le côté gauche, est généralement marbré de
brun et de jaune, enfin des troupes d'admirables mulles-rougets,
véritables paradisiers de l'Océan, que les Romains payaient jusqu'à
dix mille sesterces la pièce, et qu'ils faisaient mourir sur leur table,
pour suivre d'un œil cruel leurs changements de couleurs depuis le
rouge cinabre de la vie jusqu'au blanc pâle de la mort.

Et si je ne pus observer ni miralets, ni balistes, ni tétrodons, ni
hippocampes, ni jouans, ni centrisques, ni blennies, ni surmulets, ni
labres, ni éperlans, ni exocets, ni anchois, ni pagels, ni bogues, ni
orphes, ni tous ces principaux représentants de l'ordre des
pleuronectes, les limandes, les flez, les plies, les soles, les carrelets,
communs à l'Atlantique et à la Méditerranée, il faut en accuser la
vertigineuse vitesse qui emportait le Nautilus à travers ces eaux
opulentes.
Quant aux mammifères marins, je crois avoir reconnu en passant
à l'ouvert de l'Adriatique, deux ou trois cachalots, munis d'une
nageoire dorsale du genre des physétères, quelques dauphins du
genre des globicéphales, spéciaux à la Méditerranée et dont la partie
antérieure de la tête est zébrée de petites lignes claires, et aussi une
douzaine de phoques au ventre blanc, au pelage noir, connus sous le
nom de moines et qui ont absolument l'air de Dominicains longs de
trois mètres.
Pour sa part, Conseil croit avoir aperçu une tortue large de six
pieds, ornée de trois arêtes saillantes dirigées longitudinalement. Je
regrettai de ne pas avoir vu ce reptile, car, à la description que m'en
fit Conseil, je crus reconnaître le luth qui forme une espèce assez
rare. Je ne remarquai, pour mon compte, que quelques cacouannes
à carapace allongée.
Quant aux zoophytes, je pus admirer, pendant quelques instants,
une admirable galéolaire orangée qui s'accrocha à la vitre du
panneau de bâbord; c'était un long filament tenu, s'arborisant en
branches infinies et terminée par la plus fine dentelle qu'eussent
jamais filée les rivales d'Arachné. Je ne pus, malheureusement,
pêcher cet admirable échantillon, et aucun autre zoophyte
méditerranéen ne se fût sans doute offert à mes regards, si le
Nautilus, dans la soirée du 16, n'eût singulièrement ralenti sa
vitesse. Voici dans quelles circonstances.
Nous passions alors entre la Sicile et la côte de Tunis. Dans cet
espace resserré entre le cap Bon et le détroit de Messine, le fond de

la mer remonte presque subitement. Là s'est formée une véritable
crête sur laquelle il ne reste que dix-sept mètres d'eau, tandis que
de chaque côté la profondeur est de cent soixante-dix mètres. Le
Nautilus dut donc manœuvrer prudemment à fin de ne pas se
heurter contre cette barrière sous-marine.
Je montrai à Conseil, sur la carte de la Méditerranée,
l'emplacement qu'occupait ce long récif.
«Mais, n'en déplaise à monsieur, fit observer Conseil, c'est
comme un isthme véritable qui réunit l'Europe à l'Afrique.
—Oui, mon garçon, répondis-je, il barre en entier le détroit de
Lybie, et les sondages de Smith ont prouvé que les continents
étaient autrefois réunis entre le cap Boco et le cap Furina.
—Je le crois volontiers, dit Conseil.
—J'ajouterai, repris-je, qu'une barrière semblable existe entre
Gibraltar et Ceuta, qui, aux temps géologiques, fermait
complétement la Méditerranée.
—Eh! fit Conseil, si quelque poussée volcanique relevait un jour
ces deux barrières au-dessus des flots!
—Ce n'est guère probable, Conseil.
—Enfin, que monsieur me permette d'achever, si ce phénomène
se produisait, ce serait fâcheux pour monsieur de Lesseps, qui se
donne tant de mal pour percer son isthme!
—J'en conviens, mais, je te le répète, Conseil, ce phénomène ne
se produira pas. La violence des forces souterraines va toujours
diminuant. Les volcans, si nombreux aux premiers jours du monde,
s'éteignent peu à peu; la chaleur interne s'affaiblit, la température
des couches inférieures du globe baisse d'une quantité appréciable
par siècle, et au détriment de notre globe, car cette chaleur, c'est sa
vie.

—Cependant, le soleil...
—Le soleil est insuffisant, Conseil. Peut-il rendre la chaleur à un
cadavre?
—Non, que je sache.
—Eh bien, mon ami, la terre sera un jour ce cadavre refroidi. Elle
deviendra inhabitable et sera inhabitée comme la lune, qui depuis
longtemps a perdu sa chaleur vitale.
—Dans combien de siècles? demanda Conseil.

Le fond était encombré de sinistres épaves.
(Page 276.)
—Dans quelques centaines de mille ans, mon garçon.
—Alors, répondit Conseil, nous avons le temps d'achever notre
voyage, si toutefois Ned Land ne s'en mêle pas!»
Et Conseil, rassuré, se remit à étudier le haut fond que le
Nautilus rasait de près avec une vitesse modérée.

Là, sous un sol rocheux et volcanique, s'épanouissait toute une
flore vivante, des éponges, des holoturies, des cydippes hyalines
ornées de cyrrhes rougeâtres et qui émettaient une légère
phosphorescence, des beroës, vulgairement connus sous le nom de
concombres de mer et baignés dans les miroitements d'un spectre
solaire, des comatules ambulantes, larges d'un mètre, et dont la
pourpre rougissait les eaux, des euryales arborescentes de la plus
grande beauté, des pavonacées à longues tiges, un grand nombre
d'oursins comestibles d'espèces variées, et des actinies vertes au
tronc grisâtre, au disque brun, qui se perdaient dans leur chevelure
olivâtre de tentacules.
Conseil s'était occupé plus particulièrement d'observer les
mollusques et les articulés, et bien que la nomenclature en soit un
peu aride, je ne veux pas faire tort à ce brave garçon en omettant
ses observations personnelles.
Dans l'embranchement des mollusques, il cite de nombreux
pétoncles pectiniformes, des spondyles pieds-d'âne qui s'entassaient
les uns sur les autres, des donaces triangulaires, des hyalles
tridentées, à nageoires jaunes et à coquilles transparentes, des
pleurobranches orangés, des œufs pointillés ou semés de points
verdâtres, des aplysies connues aussi sous le nom de lièvres de mer,
des dolabelles, des acères charnus, des ombrelles spéciales à la
Méditerranée, des oreilles de mer dont la coquille produit une nacre
très-recherchée, des pétoncles flammulés, des anomies que les
Languedociens, dit-on, préfèrent aux huîtres, des clovis si chers aux
Marseillais, des praïres doubles, blanches et grasses, quelques-uns
de ces clams qui abondent sur les côtes de l'Amérique du Nord et
dont il se fait un débit si considérable à New-York, des peignes
operculaires de couleurs variées, des lithodonces enfoncées dans
leurs trous et dont je goûtais fort le goût poivré, des vénéricardes
sillonnées dont la coquille à sommet bombé présentait des côtes
saillantes, des cynthies hérissées de tubercules écarlates, des
carniaires à pointe recourbée et semblables à de légères gondoles,
des féroles couronnées, des atlantes à coquilles spiraliformes, des

thétys grises, tachetées de blanc et recouvertes de leur mantille
frangée, des éolides semblables à de petites limaces, des cavolines
rampant sur le dos, des auricules et entre autres l'auricule myosotis,
à coquille ovale, des scalaires fauves, des littorines, des janthures,
des cinéraires, des pétricoles, des lamellaires, des cabochons, des
pandores, etc.
Quant aux articulés, Conseil les a, sur ses notes, très-justement
divisés en six classes, dont trois appartiennent au monde marin. Ce
sont les classes des crustacés, des cirrhopodes et des annélides.
Les crustacés se subdivisent en neuf ordres, et le premier de ces
ordres comprend les décapodes, c'est-à-dire les animaux dont la tête
et le thorax sont le plus généralement soudés entre eux, dont
l'appareil buccal est composé de plusieurs paires de membres, et qui
possèdent quatre, cinq ou six paires de pattes thoraciques ou
ambulatoires. Conseil avait suivi la méthode de notre maître Milne
Edwards, qui fait trois sections des décapodes: les brachyoures, les
macroures et les anomoures. Ces noms sont légèrement barbares,
mais ils sont justes et précis. Parmi les macroures, Conseil cite des
amathies dont le front est armé de deux grandes pointes
divergentes, l'inachus scorpion, qui,—je ne sais pourquoi,—
symbolisait la sagesse chez les Grecs, des lambres-masséna, des
lambres-spinimanes, probablement égarés sur ce haut-fond, car
d'ordinaire ils vivent à de grandes profondeurs, des xhantes, des
pilumnes, des rhomboïdes, des calappiens granuleux,—très-faciles à
digérer, fait observer Conseil,—des corystes édentés, des ébalies,
des cymopolies, des dorripes laineuses, etc. Parmi les macroures,
subdivisés en cinq familles, les cuirassés, les fouisseurs, les
astaciens, les salicoques et les ochyzopodes, il cite des langoustes
communes, dont la chair est si estimée chez les femelles, des
scyllares-ours ou cigales de mer, des gébies riveraines, et toutes
sortes d'espèces comestibles, mais il ne dit rien de la subdivision des
astaciens qui comprend les homards, car les langoustes sont les
seuls homards de la Méditerranée. Enfin, parmi les anomoures, il vit
des drocines communes, abritées derrière cette coquille abandonnée

dont elles s'emparent, des homoles à front épineux, des bernard-
l'hermite, des porcellanes, etc.
Là s'arrêtait le travail de Conseil. Le temps lui avait manqué pour
compléter la classe des crustacés par l'examen des stomapodes, des
amphipodes, des homopodes, des isopodes, des trilobites, des
branchiapodes, des ostracodes et des entomostracées. Et pour
terminer l'étude des articulés marins, il aurait dû citer la classe des
cyrrhopodes qui renferme les cyclopes, les argules, et la classe des
annélides qu'il n'eût pas manqué de diviser en tubicoles et en
dorsibranches. Mais le Nautilus, ayant dépassé le haut fond du
détroit de Libye, reprit dans les eaux plus profondes sa vitesse
accoutumée. Dès lors plus de mollusques, plus d'articulés, plus de
zoophytes. A peine quelques gros poissons qui passaient comme des
ombres.
Pendant la nuit du 16 au 17 février, nous étions entrés dans ce
second bassin méditerranéen, dont les plus grandes profondeurs se
trouvent par trois mille mètres. Le Nautilus, sous l'impulsion de son
hélice, glissant sur ses plans inclinés, s'enfonça jusqu'aux dernières
couches de la mer.
Là, à défaut des merveilles naturelles, la masse des eaux offrit à
mes regards bien des scènes émouvantes et terribles. En effet, nous
traversions alors toute cette partie de la Méditerranée si féconde en
sinistres. De la côte algérienne aux rivages de la Provence, que de
navires ont fait naufrage, que de bâtiments ont disparu! La
Méditerranée n'est qu'un lac, comparée aux vastes plaines liquides
du Pacifique, mais c'est un lac capricieux, aux flots changeants,
aujourd'hui propice et caressant pour la frêle tartane qui semble
flotter entre le double outre-mer des eaux et du ciel, demain, rageur,
tourmenté, démonté par les vents, brisant les plus forts navires de
ses lames courtes qui les frappent à coups précipités.
Ainsi, dans cette promenade rapide à travers les couches
profondes, que d'épaves j'aperçus gisant sur le sol, les unes déjà
empâtées par les coraux, les autres revêtues seulement d'une

couche de rouille, des ancres, des canons, des boulets, des
garnitures de fer, des branches d'hélice, des morceaux de machines,
des cylindres brisés, des chaudières défoncées, puis des coques
flottant entre deux eaux, celles-ci droites, celles-là renversées.
De ces navires naufragés, les uns avaient péri par collision, les
autres pour avoir heurté quelque écueil de granit. J'en vis qui
avaient coulé à pic, la mâture droite, le gréement raidi par l'eau. Ils
avaient l'air d'être à l'ancre dans une immense rade foraine et
d'attendre le moment du départ. Lorsque le Nautilus passait entre
eux et les enveloppait de ses nappes électriques, il semblait que ces
navires allaient le saluer de leur pavillon et lui envoyer leur numéro
d'ordre! Mais non, rien que le silence et la mort sur ce champ des
catastrophes!
J'observai que les fonds méditerranéens étaient plus encombrés
de ces sinistres épaves à mesure que le Nautilus se rapprochait du
détroit de Gibraltar. Les côtes d'Afrique et d'Europe se resserrent
alors, et dans cet étroit espace, les rencontres sont fréquentes. Je
vis là de nombreuses carènes de fer, des ruines fantastiques de
steamers, les uns couchés, les autres debout, semblables à des
animaux formidables. Un de ces bateaux aux flancs ouverts, sa
cheminée courbée, ses roues dont il ne restait plus que la monture,
son gouvernail séparé de l'étambot et retenu encore par une chaîne
de fer, son tableau d'arrière rongé par les sels marins, se présentait
sous un aspect terrible! Combien d'existences brisées dans son
naufrage! Combien de victimes entraînées sous les flots! Quelque
matelot du bord avait-il survécu pour raconter ce terrible désastre,
ou les flots gardaient-ils encore le secret de ce sinistre? Je ne sais
pourquoi, il me vint à la pensée que ce bateau enfoui sous la mer
pouvait être l'Atlas, disparu corps et biens depuis une vingtaine
d'années, et dont on n'a jamais entendu parler! Ah! quelle sinistre
histoire serait à faire que celle de ces fonds méditerranéens, de ce
vaste ossuaire, où tant de richesses se sont perdues, où tant de
victimes ont trouvé la mort!

Cependant, le Nautilus, indifférent et rapide, courait à toute
hélice au milieu de ces ruines. Le 18 février, vers trois heures du
matin, il se présentait à l'entrée du détroit de Gibraltar.
Là existent deux courants: un courant supérieur, depuis
longtemps reconnu, qui amène les eaux de l'Océan dans le bassin de
la Méditerranée; puis un contre-courant inférieur, dont le
raisonnement a démontré aujourd'hui l'existence. En effet, la somme
des eaux de la Méditerranée, incessamment accrue par les flots de
l'Atlantique et par les fleuves qui s'y jettent, devrait élever chaque
année le niveau de cette mer, car son évaporation est insuffisante
pour rétablir l'équilibre. Or, il n'en est pas ainsi, et on a dû
naturellement admettre l'existence d'un courant inférieur qui par le
détroit de Gibraltar verse dans le bassin de l'Atlantique le trop plein
de la Méditerranée.
Fait exact, en effet. C'est de ce contre-courant que profita le
Nautilus. Il s'avança rapidement par l'étroite passe. Un instant je pus
entrevoir les admirables ruines du temple d'Hercule enfoui, au dire
de Pline et d'Avienus, avec l'île basse qui le supportait, et quelques
minutes plus tard nous flottions sur les flots de l'Atlantique.

CHAPITRE VIII
LA BAIE DE VIGO.
L'Atlantique! vaste étendue d'eau dont la superficie couvre vingt-
cinq millions de milles carrés, longue de neuf mille milles sur une
largeur moyenne de deux mille sept cents. Importante mer presque
ignorée des anciens, sauf peut-être des Carthaginois, ces Hollandais
de l'antiquité, qui dans leurs pérégrinations commerciales suivaient
les côtes ouest de l'Europe et de l'Afrique! Océan dont les rivages
aux sinuosités parallèles embrassent un périmètre immense, arrosé
par les plus grands fleuves du monde, le Saint-Laurent, le Mississipi,
l'Amazone, la Plata, l'Orénoque, le Niger, le Sénégal, l'Elbe, la Loire,
le Rhin, qui lui apportent les eaux des pays les plus civilisés et des
contrées les plus sauvages! Magnifique plaine, incessamment
sillonnée par les navires de toutes les nations, abritée sous tous les
pavillons du monde, et que terminent ces deux pointes terribles,
redoutées des navigateurs, le cap Horn et le cap des Tempêtes!
Le Nautilus en brisait les eaux sous le tranchant de son éperon,
après avoir accompli près de dix mille lieues en trois mois et demi,
parcours supérieur à l'un des grands cercles de la terre. Où allions-
nous maintenant, et que nous réservait l'avenir?
Le Nautilus, sorti du détroit de Gibraltar, avait pris le large. Il
revint à la surface des flots, et nos promenades quotidiennes sur la
plate-forme nous furent ainsi rendues.
J'y montai aussitôt accompagné de Ned Land et de Conseil. A
une distance de douze milles apparaissait vaguement le cap Saint-
Vincent qui forme la pointe sud-ouest de la péninsule hispanique. Il
ventait un assez fort coup de vent du sud. La mer était grosse,

houleuse. Elle imprimait de violentes secousses de roulis au Nautilus.
Il était presque impossible de se maintenir sur la plate-forme que
d'énormes paquets de mer battaient à chaque instant. Nous
redescendîmes donc après avoir humé quelques bouffées d'air.
Je regagnai ma chambre. Conseil revint à sa cabine; mais le
Canadien, l'air assez préoccupé, me suivit. Notre rapide passage à
travers la Méditerranée ne lui avait pas permis de mettre ses projets
à exécution, et il dissimulait peu son désappointement.
Lorsque la porte de ma chambre fut fermée, il s'assit et me
regarda silencieusement.
«Ami Ned, lui dis-je, je vous comprends, mais vous n'avez rien à
vous reprocher. Dans les conditions où naviguait le Nautilus, songer
à le quitter eût été de la folie!»
Ned Land ne répondit rien. Ses lèvres serrées, ses sourcils
froncés, indiquaient chez lui la violente obsession d'une idée fixe.
«Voyons, repris-je, rien n'est désespéré encore. Nous remontons
la côte du Portugal. Non loin sont la France, l'Angleterre, où nous
trouverions facilement un refuge. Ah! si le Nautilus, sorti du détroit
de Gibraltar, avait mis le cap au sud, s'il nous eût entraînés vers ces
régions où les continents manquent, je partagerais vos inquiétudes.
Mais, nous le savons maintenant, le capitaine Nemo ne fuit pas les
mers civilisées, et dans quelques jours, je crois que vous pourrez
agir avec quelque sécurité.»
Ned Land me regarda plus fixement encore, et desserrant enfin
les lèvres:
«C'est pour ce soir,» dit-il.
Je me redressai subitement. J'étais, je l'avoue, peu préparé à
cette communication. J'aurais voulu répondre au Canadien, mais les
mots ne me vinrent pas.

«Nous étions convenus d'attendre une circonstance, reprit Ned
Land. La circonstance, je la tiens. Ce soir, nous ne serons qu'à
quelques milles de la côte espagnole. La nuit est sombre. Le vent
souffle du large. J'ai votre parole, monsieur Aronnax, et je compte
sur vous.»
Comme je me taisais toujours, le Canadien se leva, et se
rapprochant de moi:
«Ce soir, à neuf heures, dit-il. J'ai prévenu Conseil. A ce moment-
là, le capitaine Nemo sera enfermé dans sa chambre et
probablement couché. Ni les mécaniciens, ni les hommes de
l'équipage ne peuvent nous voir. Conseil et moi, nous gagnerons
l'escalier central. Vous, monsieur Aronnax, vous resterez dans la
bibliothèque à deux pas de nous, attendant mon signal. Les avirons,
le mât et la voile sont dans le canot. Je suis même parvenu à y
porter quelques provisions. Je me suis procuré une clef anglaise
pour dévisser les écrous qui attachent le canot à la coque du
Nautilus. Ainsi tout est prêt. A ce soir.
—La mer est mauvaise, dis-je.
—J'en conviens, répond le Canadien, mais il faut risquer cela. La
liberté vaut qu'on la paye. D'ailleurs, l'embarcation est solide, et
quelques milles avec un vent qui porte ne sont pas une affaire. Qui
sait si demain nous ne serons pas à cent lieues au large? Que les
circonstances nous favorisent, et, entre dix et onze heures, nous
serons débarqués sur quelque point de la terre ferme ou morts.
Donc, à la grâce de Dieu et à ce soir!»
Sur ce mot, le Canadien se retira, me laissant presque abasourdi.
J'avais imaginé que, le cas échéant, j'aurais eu le temps de réfléchir,
de discuter. Mon opiniâtre compagnon ne me le permettait pas. Que
lui aurais-je dit, après tout? Ned Land avait cent fois raison. C'était
presque une circonstance, il en profitait. Pouvais-je revenir sur ma
parole et assumer cette responsabilité de compromettre dans un
intérêt tout personnel l'avenir de mes compagnons? Demain, le

capitaine Nemo ne pouvait-il pas nous entraîner au large de toutes
terres?
En ce moment, un sifflement assez fort m'apprit que les
réservoirs se remplissaient, et le Nautilus s'enfonça sous les flots de
l'Atlantique.
Je demeurai dans ma chambre. Je voulais éviter le capitaine pour
cacher à ses yeux l'émotion qui me dominait. Triste journée que je
passai ainsi, entre le désir de rentrer en possession de mon libre
arbitre et le regret d'abandonner ce merveilleux Nautilus, laissant
inachevées mes études sous-marines! Quitter ainsi cet océan, «mon
Atlantique,» comme je me plaisais à le nommer, sans en avoir
observé les dernières couches, sans lui avoir dérobé ces secrets que
m'avaient révélés les mers des Indes et du Pacifique! Mon roman me
tombait des mains dès le premier volume, mon rêve s'interrompait
au plus beau moment! Quelles heures mauvaises s'écoulèrent ainsi,
tantôt me voyant en sûreté, à terre, avec mes compagnons, tantôt
souhaitant, en dépit de ma raison, que quelque circonstance
imprévue empêchât la réalisation des projets de Ned Land.
Deux fois je vins au salon. Je voulais consulter le compas. Je
voulais voir si la direction du Nautilus nous rapprochait, en effet, ou
nous éloignait de la côte. Mais non. Le Nautilus se tenait toujours
dans les eaux portugaises. Il pointait au nord en prolongeant les
rivages de l'Océan.

Le temple d'Hercule. (Page 277.)
Il fallait donc en prendre son parti et se préparer à fuir. Mon
bagage n'était pas lourd. Mes notes, rien de plus.
Quant au capitaine Nemo, je me demandai ce qu'il penserait de
notre évasion, quelles inquiétudes, quels torts peut-être elle lui
causerait, et ce qu'il ferait dans le double cas où elle serait ou
révélée ou manquée! Sans doute je n'avais pas à me plaindre de lui,

au contraire. Jamais hospitalité ne fut plus franche que la sienne. En
le quittant, je ne pouvais être taxé d'ingratitude. Aucun serment ne
nous liait à lui. C'était sur la force des choses seule qu'il comptait et
non sur notre parole pour nous fixer à jamais auprès de lui. Mais
cette prétention hautement avouée de nous retenir éternellement
prisonniers à son bord justifiait toutes nos tentatives.
L'amiral incendia et saborda ses galions. (Page 286.)

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