Electron beam therapy

electron beam therapy

1. ELECTRON BEAM THERAPY DR ABANI KANTA NANDA 1 ST YEAR PG STUDENT DEPT. OF RADIOTHERAPY AHRCC, CUTTACK

2. DEFINITION Electron Beam Therapy is a modality of radiation therapy where the high energy electrons of energy range 6-20MeV used for treating superficial tumor <5cm deep .

3. Skin: Lip Ear Eyelids Scalp Nose limbs Upper-respiratory and digestive tract: floor of mouth soft palate retromolar trigone salivary glands Breast: chest-wall irradiation following mastectomy nodal irradiation boost to the surgical bed Other sites: Retina O rbit spine ( craniospinal irradiation) pancreas and other abdominal structures ( intraoperative therapy) cervix ( intracavitary irradiation) Electrons are useful in treating cancer of

4. WHY UNIQUE ? Distinct advantages over Superficial XRay Brachytherapy Tangential photon beam Minimising dose to deeper tissue Dose uniformity Hence G.H. Fletcher said- There is no alternative to electron beam therapy.

5. ELECTRON INTERACTION Inelastic collision with electron Ionisation and excitation Inelastic collision with atomic nuclei Bremsstrahlung Elastic collision with electron Electron electron scattering Elastic collision with atomic nuclei Nuclear scattering

6. An electron travelling in a medium loses energy as a result of collisional and radiative processes Collisional loses (ionisation and excitation) The rate of energy loss depends on the electron density of the medium The rate of energy loss per gram per centimetre squared, which is called mass stopping power , is greater for low atomic number(z) material Radiation loses ( bremsstrahlung ) The rate of energy loss per centimetre is approximately propertional to the electron energy and to the square of atomic number(z²).

7. ELECTRON SCATTERING When a beam of electrons passes through a medium, the electrons suffer multiple scattering due to interactions between the incident electrons and the nuclei of the medium. So the electrons acquire velocity components and displacements transverse to their original direction of motion. The scattering power varies Directly as the square of the atomic number inversely as the square of the kinetic energy For this reason, high atomic number materials are used in the construction of scattering foils.

8. DOSE DISTRIBUTION IN WATER ELECTRON VS PHOTON

9. DEPTH DOSE Depth in centimetre at which electron deliver a dose to the 80 -90% isodose level is equal to approximately one third to one fourth of electron energy in MeV . Most useful treatment depth or theraputic range of electron is given by depth of 90% depth dose The principle is that when in doubt , use a high energy electron to make sure that the target volume is well with in the specified isodose curve

10. Rp IS THE PRACTICAL RANG R50 IS THE DEPTH AT WHICH THE DOSE IS 50% OF THE MAXIMUM DOSE

11. MEAN ENERGY- IT HAS BEEN SHOWN THAT THE MEAN ENERGY OF THE ELECTRON BEAM, E0, AT THE PHANTOM SURFACE IS RELATED TO R50 BY THE FOLLOWING RELATIONSHIP: E0=C4 . R50 (WHERE C4 = 2.33 MeV /CM FOR WATER) ENERGY AT DEPTH- THE MEAN ENERGY OF THE SPECTRUM DECREASE LINEARLY WITH DEPTH. THIS CAN BE EXPRESSED BY THE RELATIONSHIPS: WHERE Z IS THE DEPTH

12. ENERGY DEPENDENCY ON DEPTH DOSE Percent depth dose increases as energy increases. However percent surface dose for electrons increases with energy.

13. PHANTOMS Water is the standard phantom for the dosimetry of electron beams. However, it is not always possible or practical to perform dosimetry in a water phantom. It also is difficult to make measurements near the surface of water, because of its surface tension and the uncertainty in positioning the detector near the surface. For a phantom to be water equivalent for electron dosimetry it must ‘has the same electron density (number of electrons per cubic centimeter ) and the same effective atomic number as water.

14. An effective density may be assigned to a medium to give water-equivalent depth dose distribution near the therapeutic range It has recommended that the water equivalent depth or the effective density ( ρ eff ) may be estimated from the following relationship: Where R50 is the depth of 50% dose

15. Recommended values of effective density for various phantoms are given in the table:

16. DOSE DISTRIBUTION IN PATIENT The ideal irradiation condition is for electron beam to be incident normal to a flat surface with underlying homogeneous normal tissue When the angle of incidence deviates from normal ,surface becomes irregular, internal heterogenous tissue present, the qualities of dose distribution deviates from that in phantom. Internal heterogenity can change depth of beam penetration Both irregular surface and internal heterogenities create changes in side scatter equilibrium, producing volume of increased dose( hot spot ) and decreased dose( cold spot )

17. OBLIQUE INCIDENCE For obliquely incidence beams whose angle of incidence is greater than 30°, there is a significant change in shape of central axis percent depth dose. Compared with normal incidence, the oblique incident electron beam shows the following (a) an increased surface dose, (b) an increased maximum dose, (c) a decreased penetration of the therapeutic dose (R90), (d) an increased range of penetration Clinical examples- treatment of chest wall, limbs, scalp

18. SURFACE IRREGULARITIES Sharp surface irregularities produced localised hot spots and cold spots in the underlying medium due to scattering Irregular skin surfaces are encountered primarily during treatment of nose, eye, ear, ear canal and in groin area Surgical excision creates treatment area with abrupt changes in surface in body

19. TISSUE HETEROGENEITY Dose distribution can be significantly alter in presence of tissue inhomogeneities such as bone, lung and air cavities. Electron depth dose distribution in a medium are depends on electron densities( electron/cm³) For beam passing through lung material of densities 0.25g/cm³ depth of penetration in lung would be Z(lung)=z(water)* ρ (lung) (Z= Depth of penetration, ρ =density) Thus a beam that would penetrate 1cm normal unit densities material such as water, would penetrate 4cm depth in lung.

20. Left figure shows beam incidence on chest wall with out taking the density of lung into account Right figure shows the dramatic increase in dose to lung when this inhomogeneity is taken into account (For simplicity the effect of ribs have not been considered )

21. TREATMENT PLANNING IN ELECTRON BEAM The first step in the initiation of electron therapy is to determine accurately the target to be treated. The electron energy for treatment should be selected such that the depth of 90% isodose line covers the deepest portion of the region to be treated in addition to an approximate 5mm additional depth beyond treatment region

22. Isodose curves are lines passing through points of equal dose. Isodose curves are usually drawn at regular intervals of absorbed dose and are expressed as a percentage of the dose at a reference point, which is normally taken as the Zmax point on the beam central axis. As an electron beam penetrates a medium, the beam expands rapidly below the surface, due to scattering. However, the individual spread of the isodose curves varies depending on the isodose level, energy of the beam, field size and beam collimation.

23. ELECTRON APPLICATORS Electron beam applicators or cones are usually used to collimate the beam , and are attached to the treatment unit head.

24. Normally the photon beam collimators on the accelerator are too far from the patient to be effective for electron field shaping. After passing through the scattering foil, the electrons scatter sufficiently with the other components of the accelerator head, and in the air between the exit window and the patient, to create a clinically unacceptable penumbra. Hence electron applicators are used. Several cones are provided, usually in square field sizes ranging from 5 × 5 cm² to 25 × 25 cm².

25. For a more customized field shape, a lead or metal alloy cut-out may be constructed and placed on the applicator as close to the patient as possible. Standard cut-out shapes may be preconstructed and ready for use at the time of treatment. Custom cut-out shapes may also be designed for patient treatment. Field shapes may be determined from conventional or virtual simulation, but are most often prescribed clinically by the physician prior to the first treatment.

26. INTERNAL SHIELDING For certain treatments, such as treatments of the lip, buccal mucosa, eyelids or ear lobes, it may be advantageous to use an internal shield to protect the normal structures beyond the target volume. Lead is the most common material used for production of internal shield because of its availability and ease of use

27. The required thickness of the shield depends on energy of electron beam, the fact that electron decrease in energy by 2MeV/cm in muscle 1mm of lead is required as shielding for every 2MeV of energy (plus 1mm for safety) Thus 9MeV of electrons are used to treat the buccal mucosa of thickness 1cm, a shield placed beneath the cheek to protect oral cavity would have to be 4.5mm thick. This is because the electrons would decrease to 7MeV after penetrating 1cm of tissue, and 3.5+1=4.5mm of lead would be required to shield 7MeV electrons

28. Aluminium or acrylic materials have been used around lead shields to absorb the backscattered electrons. Often, these shields are dipped in wax to form a 1 or 2 mm coating around the lead. This not only protects the patient from the toxic effects of the lead, but also absorbs any scattered electrons, which are usually low in energy

29. BOLUS Bolus is often used in electron beam therapy for the following purposes. To increase the surface dose; To flatten out irregular surfaces; To reduce the electron beam penetration in Some parts of the treatment field. Several tissue equivalent materials are used for bolus such as – paraffin wax polystyrene acrylic( pmma ) super stuff superflap superflex

30. Construction of a custom bolus to comform isodose lines to the shape of the target

31. Bolus can also be used to shape isodose lines to conform to tumour shapes. Sharp surface irregularities, where the electron beam may be incident tangentially, give rise to a complex dose distribution with hot and cold spots. Bolus around the irregularity may be used to smooth out the surface and reduce the dose inhomogeneity . The use of bolus for electron beam treatments is very practical, since treatment planning software for electron beams is limited and empirical data are normally collected only for standard beam geometries.

32. FIELD ABUTMENT The purpose of field abutment usually is to enlarge the radiation field or to change the beam energy or modality

33. This figure shows a comparison of several abutting beam configuration Fig-a—The extent and magnitude of the high dose region can be minimised by angling the central axis of each away from each other so that a common beam angle is formed Fig b—Represents overlap that can occur when the central axis of beams are parallel Fig c—Shows converging beam central axes that result in the greatest amount of overlap with the highest doses and largest high dose regions

34. ELECTRON ARC THERAPY Electron arc therapy is a special radio therapeutic technique in which a rotational electron beam is used to treat superficial tumour volumes that follow curved surfaces. While the technique is well known and accepted as clinically useful in the treatment of certain tumours, it is not widely used because it is relatively complicated and its physical characteristics are poorly understood.

35. The calculation of dose distributions in electron arc therapy is a complicated procedure and usually cannot be performed reliably with the algorithms used for standard stationary electron beam treatment planning. It is useful in treating postmastectomy chest wall, usually in barrel chested women where tangent beam can irradiate too much lung.

36. There are three levels of collimation in electron arc therapy: the primary x-ray collimators a shaped secondary cerrobend insert skin collimation The treatment-planning process for arc therapy requires patient CT scanning delineation of PTV selection of isocenter location specification of electron arc boundaries energy and slab bolus selection design of secondary collimator calculation of dose

37. INTRACAVITARY IRRADIATION It is performed for treatment of intraoral (such as floor of mouth, tongue, soft palate, retromolar trigone ) transvaginal areas Intraoperative radiotherapy is also an intracavitary irradiation For intracavitary irradiation specially design cones and adaptors are required cones cone with adaptor

38. INTRAOPERATIVE ELECTRON BEAM RADIATION THERAPY(IOERT) IOERT is a treatment option that delivers a concentrated, precise dose of radiation during surgery, immediately after a tumor is removed. Intraoperative treatments are performed completely in the operating room. Benefits One to two minutes IOERT can reduce the likelyhood of tumor recurrence and increase cure rates. It also lessen the need for post operative external radiotherapy

39. TOTAL-LIMB IRRADIATION It may be advantageous to irradiate the superficial anatomy of a limb for management of cancer , e.g., melanoma lymphoma Kaposi’s sarcoma If the depth beneath the surface is 2 cm or less, electrons offer a uniform dose while sparing deep tissues and structures. six equally spaced 5-MeV electron beams are used to irradiate a 9cm diameter cylinder.

40. TOTAL SKIN IRRADIATION Total-skin electron irradiation is a modality designed for management of diseases that require irradiation of the entire skin surface or a significant portion of it. The technique is used most frequently for treatment of mycosis fungoides kaposi’s sarcoma. Multiple techniques for total-skin electron therapy have been reviewed. The underlying principles of the various techniques are similar, which are exemplified by the modified stanford technique .

41. Schematic of modified Stanford technique . Side view of setup shows the relative position of patient plane, scatter plate, isocenter , and gantry angles (A).Six beam directions (B) are achieved by placing the patient in six patient positions (C)

42. TOTAL SCALP IRRADIATION Total-scalp irradiation is sometimes necessary in the management of malignancies (e.g., cutaneous lymphoma , melanoma , and angiosarcoma ) that present with widespread involvement of the scalp and forehead. Electron-beam therapy is a practical means of achieving the therapeutic goal of delivering a uniform dose to the scalp with minimal dose to underlying brain.

43. SUMMARY Electron beam therapy is used for treating superficial tumor . It’s benefits are minimum dose to deeper tissue and dose uniformity. Water or other water equivalent phantoms are used to study properties of electron beam therapy. Oblique incidence, irregular surface and tissue heterogeneity must be consider while planning for electron beam therapy. Electron applicators, standard cut outs or custom cut outs are used in Linac for electron beam therapy

44. Always use a high energy electron to make sure that the target volume is well within the specified isodose curve Electron arc therapy, intracavitary irradiation, total limb irradiation, total skin irradiation and total scalp irradiation are modalities of treatments where electron beams are used to treat superficial tumors .

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