3 d crt

3DCRT

1. THREE-DIMENSIONAL
CONFORMAL RADIATION
THERAPY
DR KIRAN KUMAR BR

2. 3-D CRT
oThree-dimensional conformal radiotherapy (3-D CRT)
treatments that are based on 3-D anatomic
information.
oDose distributions that conform as closely as possible
to the target volume in terms of adequate dose to the
tumor and minimum possible dose to normal tissue.
oThe concept of conformal dose distribution has also
been extended to include clinical objectives such as
maximizing tumor control probability (TCP) and
minimizing normal tissue complication probability
(NTCP).

3. PLAN TO THE 3-DCRT
Patient positioning
and Immobilization
Volumetric Data
acqusition
Image Transfer
to the TPS
Target volume
Delineation
3D Model
Generation
Forward
Planning
Dose distribution
Analysis
Treatment
Verification
Treatment
Delivery

4. Treatment-Planning Process
A . Imaging Data
oAnatomic images of high quality are required to
accurately delineate target volumes and normal
structures.
Computed tomography (CT)
Magnetic resonance imaging (MRI)
Ultrasound (US)
Single photon emission tomography (SPECT)
Positron emission tomography (PET)
oCT and MRI are the most commonly used procedures.

5. Computed Tomography
o3-D treatment planning is the ability to reconstruct images in planes
other than that of the original transverse image.
oThese are called the digitally reconstructed radiographs (DRRs).
oTo obtain high-quality DRRs images should have of high contrast and
resolution.
oThe slice thickness must be sufficiently small.
oA slice thickness of 2 to 10 mm is commonly used .

6. oTreatment planning requires patient positioning,
immobilization, and external markings that are visible
in the images.
oThe CT couch must be flat and the patient must be set
up in the same position as for actual treatment.
oImmobilization devices are essential for 3-D CRT and
should be the same as those for CT as used in the
treatment.
oFiducialpoints marked on the patient's skin or masks
should be visible in the CT images by using radiopaque
markers.
Indexer
Rod
Simulator
couch

7. oCT images can be processed to generate DRRs in any
plane, by CT simulation.
oA CT simulator is a CT scanner equipped with some
additional hardware such as
•Laser localizers to set up the treatment isocenter
•A flat table or couch insert, and
•Image registration devices.
A computer workstation with special software to
process CT data, plan beam directions, and generate
BEV DRRs allows CT simulation films with the same
geometry as the treatment beams.

8. Magnetic Resonance Imaging
oIn treatment planning, MRI images may be used alone or in
conjunction with CT images.
oMRI is considered superior to CT in soft-tissue discrimination.
oMRI is well suited to imaging head and neck cancers, sarcomas,
the prostate gland, and lymph nodes.
oOn the other hand, it is insensitive to calcification and bony
structures, which are best imaged with CT.

9. oCT and MRI image characteristics, the two are
considered complementary in their roles in treatment
planning.
oThe most basic difference between CT and MRI is that
the former is related to electron density and atomic
number .
oThe best spatial resolution of both modalities is similar
(~1 mm).
oMRI takes much longer than CT ,is susceptible to
artifacts from patient movement.
oMRI can be used to directly generate scans in axial,
sagittal, coronal, or oblique planes

10. oCT images are considered a reference for anatomic
landmarks
oCT provides the best geometric accuracy
Digital reconstructed radiographs
created from
transverse computed tomo
graphyscan.
Image A is frontal
and image B is lateral

11. B . Image Registration
oIt is the process of correlating different image data sets to identify
corresponding structures or regions.
oFacilitates comparison of images from one study to another and
fuses them into one data set that could be used for treatment
planning.
oComputer programs are now available that allow image fusing
oFor example, mapping of structures seen in MRI onto the CT
images.

12. oVarious registration techniques include
•Point-to-point fitting
•Interactively superimposing images in the two data
sets,
•Surface or topography matching.

13. CT IMAGE
CONTOURING ON BLENDED IMAGE
MRI IMAGE
IMAGE FUSION

14. C. Image Segmentation
oThe term image segmentation in treatment planning
refers to slice-by-slice delineation of anatomic regions
of interest
oFor example, external contours, targets, critical
normal structures, anatomic landmarks, etc.
oThe segmented regions can be rendered in different
colors and can be viewed in BEV configuration or in
other planes using DRRs.

15. oSegmentation is also essential for calculating dose
volume histograms (DVHs) for the selected regions of
interest.
oIt is one of the most important processes in treatment
planning.

16. Image segmentation for
prostate gland treatment
planning. Prostate gland,
bladder, and rectum are
delineated in different
colors. Segmented
structures are shown in
transverse A: lateral B: and
coronal C: planes.

18. VOLUME DEFINITION
oVolumedefinitionisprerequisite
for3-Dtreatmentplanning.
oICRUreports50&62define&
describetarget&critical
structurevolumes.

19. ICRU 50 & 62
oWhendeliveringRadiotherapytreatment,parameters
suchasvolume&dosehavetobespecifiedfor:
Prescription
Recording
Reporting
oSuchparametersareusedforreportingpurposeto
ensureacommonlanguagebetweendifferentcenters.
oItisexpectedthatrapiddevelopmentofnewtechniques
wouldincreasethecomplexityofradiotherapyand
emphasizetheneedforgeneralstrictguidelines.

20. VOLUMES
oTwovolumesshouldbedefinedpriorto
treatmentplanning,thesevolumesare:
Grosstumorvolume(GTV).
Clinicaltargetvolume(CTV).
oDuringthetreatmentplanningprocess,
othervolumeshavetobedefined.
Planningtargetvolume(PTV).
Organsatrisk.
oAsaresultoftreatmentplanning,further
volumescanbedescribed.Theseare:
Treatedvolume(TV).
Irradiatedvolume(IRV).

21. oTheGTVisthegross(palpable,visibleor
demonstrable)extentandlocationofmalignant
growth.
oThismayconsistofprimarytumor,metastatic
lymphadenopathyorothermetastases.
oNoGTVcanbedefinedifthetumorhasbeenremoved.
Eg.Byprevioussurgery.
oTheCTVisGTV+subclinicalmicroscopicdisease.
oAdditionalvolumeswithpresumedsubclinicalspread
mayalsobeconsideredfortherapyandmaybe
designatedasCTVII,CTVIIIetc.(ICRU62).

22. oThePTVisageometricalconceptdefinedtoselect
appropriatebeamsizesandbeamarrangements.
oItconsiderstheneteffectofthegeometricalvariations
toensurethattheprescribeddoseisactuallyabsorbed
intheCTV.
oThesevariationsmaybeintra-fractionalorinter-
fractionalduetonumberoffactorslike
Movementoftissues/patient.
Variationsinsize&shapeoftissues.
Variationsinbeamcharacteristics.
Theuncertaintiesmayberandomorsystematic.

23. ORGANS AT RISK
oOrgansatriskarenormaltissues,whoseradiation
sensitivitymaysignificantlyreducethetreatment
planningand/orprescribeddose.
oAnypossiblemovementoftheorganatriskaswell
asuncertaintiesinthesetupduringthewhole
treatmentcoursemustbeconsidered.

24. oOrgansatriskmaybedividedintothree
differentclasses:
ClassI(Radiationlesionsarefatal&resultinsevere
morbidity.)
Bonemarrow,Brain,Spinalcord,Heart,Lung,Kidney,
Liver,Intestine,Stomach.
ClassII(Resultinmoderatetomildmorbidity.)
Oralcavity,Pharynx,Skin,Esophagus,Salivaryglands,
Bladder,Rectum.
ClassIII(Radiationlesionsaremild,transientand
reversibleorresultinnosignificantmorbidity.)
Lymphnodes,Largearteriesandveins,Uterus,Vagina.

25. LUNGS
SPINAL CORD

27. ICRU 62, 1999
oGivesmoredetailedrecommendationsondifferentmarginsthat
mustbeconsideredtoaccountforAnatomical&Geometrical
uncertainties.
oIntroducesconceptofreferencepoints&coordinatesystems.
oIntroducestheconceptofconformityindex.
oClassifiesOrgansatRisk.
oIntroducesplanningorganatriskvolume.
oGivesrecommendationsongraphic.

28. INTERNAL MARGIN (IM) &
INTERNAL TARGET VOLUME ( ITV)
oAmarginmustbeaddedtotheCTVtocompensateforexpected
physiologicalmovements&expectedvariationsinsize,shape&
positionoftheCTVduringtherapy.
oItisinrelationtoaninternalreferencepointanditscorresponding
coordinatesystems.
oThismarginisnowdenotedastheInternalMargin(IM).
oTheydonotdependonexternaluncertaintiesofbeamgeometry.

29. ICRU 62 report

30. SET UP MARGIN (SM)
oItaccountsfortheuncertaintiesinpatientpositioningand
aligningoftherapeuticbeams.
oItincludesthetreatmentplanningsessionaswellasall
thetreatmentsessions.PLANNING
ORGAN ATRISKVOLUME
(PRV)
oAnintegratedmarginmustbeaddedtotheORto
compensateforvariationsincludingthemovementof
organaswellassetupuncertainties.
oInparticulartheinternalmargin&thesetupmarginfor
theORmustbeidentified.Thisleadstotheconceptof
PRV.

32. D. Beam Aperture Design
oAfter image segmentation , the task is to selecting beam direction
and designing beam apertures.
oThis is greatly aided by the BEV capability of the 3-D treatment-
planning system.
oTargets and critical normal structures made visible in different
colors through segmentation can be viewed from different
directions in planes perpendicular to the beam's central axis.

33. oBeam apertures can be designed automatically or
manually depending on the proximity of the critical
structures and the uncertainty involved in the allowed
margins between the CTV and PTV.
oA give and take occurs between target coverage and
sparing of critical structures.
oWhere the spaces between the target and critical
structures are tight, thus requiring manual design of the
beam apertures.
oIn simpler cases, automatic margins may be assigned
between the PTV and field edges, taking into account the
field penumbra and the required minimum isodose
coverage of the PTV.

34. oGenerally, a 2-cm margin between the PTV and the
field edge ensures better than 95% isodose coverage
of the PTV, but this must be ascertained through actual
computation of the dose distribution.

35. Beam's eye view of anterior-
posterior A: and left-right
lateral B: fields used in the
treatment of prostate gland.
Composite (initial plus boost)
isodose curves for a four-field
plan are displayed in
transverse C: sagittal D: and
coronal E: planes.

36. E. Field Multiplicity and
Collimation
oThree-dimensional treatment planning encourages the use of
multiple fields.
oBecause targets and critical structures can be viewed in the BEV
configuration individually for each field.
oMultiplicity of fields also removes the need for using ultra-high-
energy beams (>10 MV).
oThe greater the number of fields, the less stringent is the
requirement on beam energy because the dose outside the PTV is
distributed over a larger volume.

37. oThe 3-D treatment planning also allows noncoplanar beam
direction.
oThe beam central axis lies in a plane other than the transverse
plane of the patient.
oTo use a noncoplanar beam, the couch is rotated (“kicked”)
through a specified angle
oMaking sure that it will not collide with the gantry.

38. oUsing a large number of fields creates the problem of
designing an excessive number of beam-shaping blocks
and requiring longer setup times.
oAn alternative method to multiple field blocking is the
use of a multi leaf collimator (MLC) .
oMLCs can be used with great ease and convenience to
shape fields electronically.
oCombination of MLCs and independent jaws provides
almost unlimited capability of designing fields of any
shape.

39. oCustom-designed blocks are still useful.
oIn treating small fields, midfield blocking (“island”
blocks), or complex field matching.
oMLCs provide a logistic solution to the problem of
designing, carrying, and storing a large number of
heavy blocks.

40. F. Plan Optimization and Evaluation
Isodose Curves and Surfaces
oTreatment plans are optimized iteratively by using
multiple fields, beam modifiers, beam weights, and
appropriate beam directions.
oDose distributions of competing plans are evaluated by
viewing isodose curves in individual slices, orthogonal
planes or 3-D isodose surfaces.

41. oThe dose distribution is usually normalized to be 100%
at the point of dose prescription.
oSo that the isodose curves represent lines of equal
dose as a percentage of the prescribed dose.
oDisplay of dose distribution in the form of isodose
curves or surfaces is useful.
oIt shows not only regions of uniform dose, high dose,
or low dose, but also their anatomic location and
extent.

42. A conformal stereotactic
treatment plan for a
pituitary tumor showing
isodose curves in the
transverse A: lateral B:
and coronal C: planes.
Prescription isodose
surfaces covering the
target volume are
displayed in frontal D: and
lateral E: planes.

43. Dose-volume Histograms
oDVH contains the 3D distribution data to graph, it
describes the radiation distribution within a specifically
defined volume of interest.
oSo the summarizing and analyzing the3D dose data is
possible.

44. oA DVH not only provides quantitative information with regard to how
much dose is absorbed in how much volume.
oIt summarizes the entire dose distribution into a single curve for each
anatomic structure of interest.
oSo it is a grate tool for plan evaluation.
•TheDVHmayberepresentedintwoforms:
•CumulativeintegralDVH
•DifferentialDVH.

45. CUMULATIVE DVH

46. oIt is plot of volume of a given structure receiving a
certain dose.
oAny point on the cumulative DVH curve shows the
volume of a given structure that receives the indicated
dose or higher.
oThe cumulative DVH has been found to be more useful
and is more commonly used than the differential form.

47. DIFFERENTIAL DVH

48. oThe direct or differential DVH is a plot of volume
receiving a dose within a specified dose interval (or
dose bin) as a function of dose.
oIt shows extent of dose variation within a given
structure.
oThe ideal DVH for a target volume would be a single
column indicating that 100% of volume receives
prescribed dose.
oFor a critical structure, the DVH may contain several
peaks indicating that different parts of the organ
receive different doses.

49. LIMITATIONS
oThe DVH shows that the hot and cold spots exist, but do not indicate
where they occur.
oDVH provide a graphical summary of the dose distribution within a
structure , they do not give spatial information.
oDVHs do not indicate the complexity of the field arrangement.

50. Dose Computation Algorithms
oDose calculation algorithms for computerized treatment planning
have been evolving since the middle of the 1950s.
oIn broad terms the algorithms fall into three categories:
(a) correction based
(b) model based
(c) direct Monte Carlo

51. CORRECTION-BASED ALGORITHMS
oThey are based primarily on measured data (e.g.,
percent depth doses and cross-beam profiles, etc.)
obtained in a cubic water phantom.
oVarious correction factors are applied to calculate
dose distributions in a patient.

52. oThe corrections typically consist of
(a) Attenuation corrections for contour irregularity
(b) Scatter corrections as a function of scattering
volume, field size, shape, and radial distance
(c) Geometric corrections for source to point of
calculation distance based on inverse square law
(d) Attenuation corrections for beam intensity
modifiers such as wedge filters, compensators, blocks,
etc.
(e) Attenuation corrections for tissue heterogeneities
based on radiologic path length (unit-density
equivalent depth).

53. oThe dose at any point is usually analyzed into primary
and scattered components, which are computed
separately and then summed to obtain the total dose.

54. Model-based Algorithms
oA model-based algorithm computes dose distribution with a physical
model that simulates the actual radiation transport.
oBecause of its ability to model primary photon energy fluence
incident at
a point and the distribution of energy subsequent to primary photon
interaction,
oIt is able to simulate the transport of scattered photons and electrons
away from the interaction site.
oA class of model-based algorithms, called convolution-
superposition,

55. Convolution-superposition Method
oA convolution-superposition method involves a convolution equation
that separately considers the transport of primary photons and that
of the scatter photon and electron emerging from the primary photon
interaction.

56. The dose ṝ at a point ṝ given by:
µ/ρis the mass attenuation coefficient
ψp(ṝ′) is the primary photon energy fluence
A(ṝ -ṝ′) is the convolution kernel
Kernelis a matrix of dose distribution deposited by
scatter photons and electrons set in motion at the
primary photon interaction site.
oThe product of mass attenuation coefficient and the
primary energy fluenceis called terma, Tp(ṝ′).

57. oIt stands for total energy released per unit mass.
oTerma is analogous to kerma.
oWhich represents the kinetic energy released per unit
mass in the form of electrons set in motion by
photons.
oThe product of terma and the dose kernel when
integrated (convolved) over a volume gives the dose
D(ṝ).
oThe convolution kernel, A(ṝ -ṝ′), can be represented
by a dose spread array obtained by calculation or by
direct measurement.

58. oThe most commonly used method is the Monte Carlo.
oWhich simulates interactions of a large number of
primary photons and determines dose deposited in all
directions by electrons and scattered photons
originating at the primary photon interaction site.
oMonte Carlo may be used to calculate the energy
spectrum of a linac beam.

59. oThe Monte Carlo–generated energy spectrum and the
kernel are essential ingredients of the convolution
equation to compute dose at any point in the patient.
oA convolution equation when modified for radiologic
path length (distance corrected for electron density
relative to water) is called the convolution-
superposition equation:

60. oρṝ· ṝ′ is the radiologic path length from the source to the primary
photon interaction site .
oρṝ -ṝ′ · (ṝ -ṝ′) is the radiologic path length from the site of primary
photon interaction to the site of dose deposition.

61. Direct Monte Carlo
oThe Monte Carlo technique consists of a computer program (MC
code) that simulates the transport of millions of photons and particles
through matter.
oIt uses fundamental laws of physics to determine probability
distributions of individual interactions of photons and particles.
oThe larger the number of simulated particles (histories), the greater
the accuracy of predicting their distributions.

62. oThe number of simulated particles is increased, the
computational time becomes prohibitively long.
oThe dose distribution is calculated by accumulating
(scoring) ionizing events in bins (voxels) that give rise
to energy deposition in the medium.
oIt is estimated that the transport of a few hundred
million to a billion histories will be required for
radiation therapy treatment planning with adequate
precision.

63. A number of MC codes has been used in radiation transport simulation
and, more recently, in treatment planning:
•Electron Gamma Shower version 4 (EGS4) (15), ETRAN/ITS (16),
Monte Carlo N-particle (MCNP) (17), PENELOPE (18), and PEREGRINE
(developed at Lawrence Livermore National Laboratory) (19).

64. IMRT
oIMRT is a radiation technique in which the intensity of the radiation
beam is modulated.
oEach beam is composed of several segments which can vary in size
and shape.

65. oFor each radiation beam/gantry angle, the segments
are formed using a multi-leaf collimator, and the
combination of them results in an intensity-modulated
dose delivery.
oThe end result of an IMRT treatment, when all
segments for every radiation beam have been
delivered, is a three dimensional, non-uniform dose
distribution which conforms to the tumor.
oThis implies high dose to tumor volumes and low dose
to surrounding normal tissue.

66. Step-and-shoot
oStep-and-shoot is a static IMRT technique in which each radiation
beam consists of several discrete segments, and the radiation is
turned off between the segments.
oFirst, the gantry and MLC leaves assume their starting positions.
oThe radiation is then turned on, and the amount of monitor units for
that specific segment is delivered by the linac.

67. oThe radiation is then turned off, the MLC leaves relocate to create the
next segment shape, and a new output of radiation is given.
oThis process is repeated for every segment within all beams/gantry
angles in the treatment plan.

68. Sliding windows
oSliding windows is also an IMRT technique with fixed gantry angles,
but what makes it different from step-and-shoot is that the segments
are delivered in a dynamic matter.
oDuring dose delivery for a specific beam/gantry angle, the radiation is
continuously on with constant intensity while the MLC leaves move
across the radiation field.

69. oAll leaves move in the same, single direction, and the velocity of
each leaf is modulated for each radiation beam/gantry angle to
obtain the desired dose distribution.

70. Volumetric modulated arc therapy
oVolumetric modulated arc therapy (VMAT) is a radiation technique in
which the gantry moves continuously around the patient, while the
radiation is constantly on.
oThe gantry has varying speed, and the dose rate (MU per time) also
varies.
oThe shape of the treatment field changes dynamically during gantry
rotation due to the varying velocities and positions of the MLC
leaves, which can move back and forth along the y-axis.

71. oThe result of a VMAT treatment is, as for IMRT, an
intensity modulated dose distribution which conforms
to the tumor volumes.
oA single rotation of up to 360◦of the gantry is defined
as an arc, and the number of arcs used for treatment is
optional.
oDepending on the aperture models used for radiation
delivery, different restrictions apply to gantry rotation,
dose rate, and speed and position of MLC leaves.

72. IGRT
oImage-guided radiation therapy (IGRT) may be defined as a radiation
therapy procedure that uses image guidance at various stages of its
process:
opatient data acquisition
otreatment planning
otreatment simulation
opatient setup
otarget localization before and during treatment.

73. oIGRT , that uses image guidance procedures for target
localization before and during treatment.
oThese procedures use imaging technology to identify and
correct problems arising from inter-and intrafractional
variations in patient setup and anatomy, including shapes
and volumes of treatment target, organs at risk, and
surrounding normal tissues.
oIn that context, we will describe a number of image
guidance technologies and methods that are available to
implement IGRT.

74. oAs the planning target volumes (PTVs) are made
increasingly conformal such as in three-dimensional
conformal radiotherapy (3-D CRT) and intensity-
modulated radiation therapy (IMRT), the accuracy
requirements of PTV localization and its dosimetric
coverage during each treatment become increasingly
stringent.
oThese requirements have propelled advances in the
area of dynamic targeting of PTV and visualization of
surrounding anatomy before and during treatments.

75. oImaging systems have been developed that are accessible in the
treatment room or mounted directly on the linear accelerator.
oThe accelerator-mounted imaging systems are called on-board
imagers (OBIs).

76. Portal and Radiographic Imagers
oModern accelerators (e.g., Varian's Trilogy) are equipped
with two kinds of imaging systems:
o(a) kilovoltage x-ray imager in which a conventional x-ray
tube is mounted on the gantry with an opposing flat-panel
image detector
o(b) megavoltage (MV) electronic portal imaging device
(EPID) with its own flat-panel image detector.
oThe flat-panel image detector in both cases is a matrix of
256 ×256 solid state detectors consisting of amorphous
silicon (a-Si).

77. oKilovoltage (kV) images have better contrast than the
MV images of the EPIDs,
oNeither of them is of sufficiently good quality to
visualize soft-tissue targets in their entirety.
oThe OBIs are quite useful in determining the planned
target position in relation to the bony landmarks
and/or radio-opaque markers (fiducials) implanted in
the target tissues.

78. oIn addition, the kV imager can be used in both the radiographic and
fluoroscopy modes to check patient setup before each treatment or
track the movement of fiducial markers due to respiratory motion.
oThe MV imager can provide portal verification before each treatment
as well as on-line monitoring of target position during treatment
delivery.

79. THANK YOU