Electron therapy - radiation physics

1. Clinical and Physical characteristics
of Megavoltage Electron Therapy
Presenter : Dr. Gowtham Manimaran
Moderator : Mr. Malhotra

2. Introduction
• Electron beam therapy (EBT) is a kind of external beam radiotherapy
where electrons are directed to a tumour site.
• Megavoltage electron beams represent an important treatment
modality in modern radiotherapy , often providing a unique option in
the treatment of superficial tumors up to a depth of about 5 cm
effectively sparing deeper normal tissues.
• Clinically useful energies are between 6 Mev and 20 Mev.

3. • Common uses: The most common clinical uses of electron beams
include the treatment of skin lesions (skin , lip , pinna), such as
basal cell carcinomas,
boosting of (giving further radiation to) areas that have previously
received photon irradiation such as the postoperative lumpectomy
cavity or mastectomy scar in breast cancer patients,
As well as select nodal areas in the head and neck.
Vulva ,perineum
CSI
IORT

4. Production
• Electrons are produced
from the cathode by
thermionic emission
• Guided and changed in
direction by bending
magnets
• Hits the scattering filter
to spread out as well as
get a uniform energy
distribution

5. Applicators

6. Inelastic collisions Elastic collisions
Electron interactions
As electrons travel through a medium, they interact with atoms by a
variety of processes owing to Coulomb force interactions. These processes
include,

7. Physical characteristics
• They can be described under the following headings,
In water phantom
Depth Dose
Isodose curves
In patient
Tissue heterogeneities

8. 1.Depth dose (in a water phantom)
• Major attraction of the electron beam irradiation is the shape of
the depth dose curve

9. Surface dose
• Surface dose for megavoltage electron beams is
relatively large (typically between 75 % and 95 %)
in contrast to the surface dose for megavoltage
photon beams which is of the order of 10 % to 25
%.
• Unlike in photon beams, the percentage surface
dose in electron beams increases with increasing
energy.

10. Bremsstrahlung contamination
• Bremsstrahlung tail (x ray contamination) forms
beyond the range of electrons and is attributed to the
bremsstrahlung photons
Less than 1 % for 4 MeV electron beams.
Less than 2.5 % for 10 MeV electron beams.
Less than 4 % for 20 MeV electron beams

11. Depth Depending on Energy
• Surface dose(Ds) increases energy increases.
• Depth of distal 90% (R90), often the therapeutic
prescription depth, increases as energy increases.
(A physician may also prescribe to the 80% or 100%)
• The practical range (Rp) (maximum penetration) of the
electrons increase as energy increases.
• X-ray dose attributable to bremsstrahlung that lies beyond
the electron dose component increases as energy
increases.

12. Selection of beam energy

13. Clinical application
R 90 approximately given by E/3.2 cm (where E is the most probable energy in
MeV of the electron beam at the surface).
R 80(80% depth dose) occurs approximately at E/2.8 cm
Example :
1.Say to treat a lumpectomy cavity of depth 2 cm and we want to prescribe 90%
and what energy of electron we should choose?
6Mev/ 3.2 = 1.875 cm , 9 Mev/3.2 = 2.81 cm
Say we want 80% dose prescription
6Mev /2.8 = 2.14 cm , 9 Mev /2.8 = 3.2 cm

15. Dependence of PDDs on electron beam field size
 The depth increases with field size
because of the increased scatter
from the collimator and the
phantom.
 Most clinical significant effect is
the decrease in R90 with
decreasing field size clinically
this correlates to requiring higher
energies for smaller fields.

16. 2.Iso dose curves
• Definition : are lines joining the points of equal percentage depth dose
• Measured by,
Ion chambers
Solid state detectors
RF

17. • For low energy electrons , isodose bulging out for all dose levels
• For high energy electron beams , isodose curves constrict for high
dose levels but bulge out for low dose levels

18. 3.Tissue heterogeneities (dose distribution in patient)
• For most clincal circumstances , the ideal irradiation condition is for
the electron beam to strike a flat surface with underlying
homogenous soft tissues
• Due to surface irregularity , internal heterogenous tissues (air , lung ,
bone) the qualities of dose distribution gets affected which may
potentially lead to PTV underdose and critical structure overdose.

19. Air cavities
• Isodose contours in the shadow of air
are shifted distally
• The dose beneath the air cavity
increases
• Influence of air increases laterally
with depth

20. • Lung:
Dose penetration in lung is 3 to 4
times more !!
• Bone:
The isodose in the shadow of
bone are shifted proximally
Dose beneath the bone
decreases by 5%

21. Dependance on angle of incidence(obliquity)
• Increased surface dose
• Increased maximum dose
• Decreased penetration of
therapeutic dose R90
• Increased range of penetration
Clinical examples : chest wall
treatments , treatment of the
limbs , treatment of the scalp

22. Irregular Surface
Sharpe surface irregularity produces localized Hot and Cold spot.
Effect are due to scattering.
Projection Causes outward scattering.
Steep depression causes inward scattering.

23. Use of Bolus
Bolus has other uses apart from
correcting for surface irregularities &
surgical defects.
Used for increasing the skin dose
If the edges of the bolus are sharp
High dose at the edges inside the
treated field
Cold spot at the edges under the bolus.
Edges Should be tempered.

24. • Place Bolus close to skin surface-If placed too far, scattered electrons
will increase penumbra & decrease the maximum dose.

25. Treatment planning in Electron beam
• 1st step is to determine accurately the target to be treated. All available diagnostic
,operative and medical information should be consulted to determine the extent and the
final planning target volume (PTV) with appropriate marigins.
• Then comes selection of beam energy- depth of the 90% -80% isodose line should cover
the distal or deepest portion of the region to be treated
• Electron collimation – either by an applicator’s collimating insert and/or skin collimation
• Need for bolus
• Shielding

26. Collimation
• Made of :Cerrobend / lead
• Clinical application for:
Small Fields
Protection of Critical Structures
Under Bolus
Electron Arc Therapy

27. Internal shielding
• This is most commonly seen when treating lip , buccal mucosa and
eyelid lesions
• Lead is the most commonly used
• Requirement : 1mm of lead is required for every 2MeV of electron
energy (plus 1mm for safety)

28. Specialised Electron Techniques
• Intracavitary Irradiation
• Total Scalp Irradiation
• Total Limb Irradiation
• Total Skin Irradiation
• Craniospinal irradiation
• Electron Arc therapy

29. Electron VS Photon

30. • Thank You!