ELECTRON BEAM RADIOTHERAPY

1. DR.K.C PATRO
ELECTRON BEAM THERAPY

2. DEFINITION
It is a type of particulate beam
radiation therapy used for the
treatment of Superficial tumors

3. BEAM
PHOTON PARTICULATE
X ray γ ray
α,β,proton,e,He,C

4. WHY ELECTRON BEAM
 No exit dose like X-ray
 Delivery of reasonably uniform dose from skin to a specific
depth
 abrupt dose fall off after 90% to 80% isodose curve to near
zero level

5. Photon BeamPit Falls we want
 Exit dose
 No 100% Skin sparing
 No 100% skin dose
 No 100% tumor control
 No normal tissue sparing
 Side scatter
 Low LET
 High OER
 Low RBE
 No exit dose
 100% Skin sparing
 100% skin dose
 100% tumor control
 Normal tissue sparing
 No Side scatter
 High LET
 Low OER
 High RBE

6. PHOTON vs. ELECTRON
 Exit dose
 Near 100% Skin sparing
 No 100% skin dose
 High penetration
 No DOSE uniformity
 No exit dose
 near 100% skin dose
 abrupt dose fall off after
80% isodose curve
 High scatter
 Dose uniformity

7. MACHINES
 Van De Graff generator.
 Betatron
 Linear accelerator – a device that uses
high frequency EM waves to accelerate
charged particles viz. electrons to a high
energy through a linear tube

9. Mechanism of production

10. X-ray Mode/ electron Mode

11. Parts of treatment head
1. Primary Collimator
2. Scattering foil
3. Flattening filter
4. Ionization chamber
5. Secondary collimator
6. Tertiary collimator ( trimmer , electron
cone, auxiliary collimator )

12. SCATTERING FOIL
1. The scattering foils are made up of high
dense material interposed in electron beam
2. Different scattering foils are used for
different e- energy
3. The electron beam edges can be sharply
defined only if the collimator extended
towards the skin of the pt by attachment of
trimmers/applicators
4. The electron beam trimmers are optimally
designed to give uniform fluency

13. ELECTRON BEAM FROM
MACHINE TO PATIENT
 The energetic electron beam emerging from
accelerators is pencil beam.it is not suitable for
treatment
 Hence the beam has to be spread to a larger area
for treatment.
 Spread can be done by- electromagnetic scattering
device or scattering foil
 To further spreading electron applicator is used

14. Mechanism of interaction of electron with matter
 Inelastic collision with atomic electron
 Inelastic collision with nuclei
 elastic collision with atomic electron
 elastic collision with nuclei

15. Fundamental questions before EBT
 Indication of EBT
 Goal of EBT
 Planned Treatment Volume
 Planned Treatment dose
 Planned Treatment Technique

16. Indications
 Treating skin cancers.
 Chest wall irradiation in Ca breast.
 Boosting to neck node after 45 Gy.
 Total skin irradiation in mycosis fungoides.
 Total limb irradiation
 Total scalp irradiation
 Craniospinal irradiation
 Intracavitary irradiation

17. Dosimetry
 Measured by pencil beam algorithm
 Accurate in water at standard and extended
SSD
 Correctly predicts changes in Penumbra
 Predict changes in dose in oblique incidence
or irregular surfaces

18. Isodose Curves

19. Isodose curve characteristic
 Rapid dose falloff below 80% isodose
 Bulging towards bottom
 Ballooning towards edge

20. Isodose curve in slopping
surface

21. Determination of absorbed dose-
 Calorimeter
 TLD
 Solid state diode
 Ionization chamber

22. Depth dose characteristic/isodose
characteristic
 There is a abrupt fall of doses beyond 90% to
80% of isodose curve
 80% or 90% isodose curve is taken as standard
dose for prescription
 Depth (cm)of 90% isodose=E(Mev)/4
 Depth (cm)of 80% isodose=E(Mev)/3
 Electron beam penetrates a finite depth with clear
cut range,the photon beam in other hand proceeds
infinite range.

23. Dose dependence on incident energy

24. Dose dependence on field size

25. Depth Dose dependence on SSD

26. dmax
 Maximum dose is not on skin
 It is somehow away from skin
 It is due to electronic equilibrium
 6Mev-10mm
 9Mev-15mm
 12Mev-19mm
Approximation

27. Range of depth
 Roughly E/2
 E=energy
 E.g –for 6Mev range is 6/2=3cm

28. Surface dose in 10x10 cm field
 For 5mev=74%
 10mev=82%
 16mev=93%
 25mev=96%
 It depends upon energy,field size and
thickness of scattering foil

29. Energy range
 6-20 Mev
6,9,12,15,18,20 mev

30. Selection of energy
Ep0(MeV) = 3.3 x R90 (cm) [R90 exceeds max
depth of PTV]
Ep0(MeV) = 2 x Rp [Rp is the practical range
of electrons ]

31. Ideal condition for EBT
 Electron beam incidence normal to flat
surface
 Underlying homogenous soft tissue
(provide uniform dose in penumbra from
surface to R90 after which there is rapid
dose fall off)

32. USE OF BOLUS
 To flatten out irregular surface
 Reduce the penetration of electrons in part of the field
 Increase the surface dose
 To act as a missing tissue compensator
 Thickness of bolus required increases as the surface dose
increases e.g
– 6-10mev=1cm
– 10-15mev=0.5cm
– >15mev=no bolus

33. BOLUS MATERIAL
 Paraffin wax,polysterene,lucite,superstoff,superflab
 Flexible bolus that confirms to surface is desirable
 Large air gap between the absorber and the surface
would result in scattering of e- outside the field and
reduction in dose that may not be easily predictable
unless specially measured for this condition

36. MLC vs. BLOCK
 No MLC
 Wooden MLC
 Wax MLC
 Customized block

37. BLOCKING MATERIAL
 Usually Cerroband
 Cerroband-Pb+Bi+Sn+Cu
 Melting point=
 Customized block
 Thickness of block required to use as block
=E/2 e.g for 6mev=6/2=3cm

38. Monitor unit
CGy/MU

39. Decelerators
 A plate of low atomic wt. material such as
lucite and polysterene is sometimes used to
reduce the energy of electron beam known
as decelerators
 It must be placed in close contact with
patient surface with bolus

40. Electron arc
therapy

41. Electron field-photon field
combination

42. Total skin irradiation-

43. Total scalp irradiation

44. Craniospinal irradiation

45. Total limb irradiation