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
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
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.
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
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
34.
35.
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
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