Electron Beam Therapy

1. Presenter: Dr.Jyotiman Nath
PGT(Radiotherapy)

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
Tumours
Clinically useful energies are between 6 and 20-MeV

3. Why Electrons?
 Delivers a reasonably uniform dose from the
surface to a specific depth , after which dose falls
off rapidly, eventually to a near-zero value.
 Using electron beams allows disease within
approximately 6 cm of the surface to be treated
effectively, sparing deeper normal tissues.

4. History of Electron Therapy
Van de Graaff Accelerators (late 1930s)
•E<3 MeV; mainly source of x-ray beams
•First used at Huntington Memorial Hospital in 1937
•Limited utilization for mycosis fungoides and other
skin cancers
Betatrons (late 1940s)
•Developed in US and Germany
•Beam line and dosimetry development

5. Linear Accelerators (1960s)
Electron beam therapy is now performed using a
medical linear accelerator.
The same device can also be used to produce
high energy photon beams

6. Clinical Utility
Electron beams have been successfully used in
numerous sites that are within 6 cm of the surface
•Cancer of skin regions, or total skin (e.g. Mycosis fungoids)
•Diseases of the limb (e.g. melanoma and lymphoma),
nodal irradiation(Neck Node Boost)
•It may also be used to boost the radiation dose to the
surgical bed after mastectomy or lumpectomy.
•For deeper regions intraoperative electron radiation
therapy can be applied.

7. Electron Interactions with matter
As electrons travel through a medium, they interact
with atoms by a variety of processes owing to Coulomb
force interactions.
The processes are..........
(a) inelastic collisions with atomic electrons
( ionization and excitation ),
(a) inelastic collisions with nuclei ( bremsstrahlung )
(b) elastic collisions with atomic electrons, and
(d) elastic collisions with nuclei

8. In inelastic collisions, some energy is lost as it is
used up in producing ionization or converted to
other forms of energy like photon energy and
excitation energy.
In elastic collision kinetic energy is not lost although
it may be redistributed among the particles
emerging from the collision.

9. Major attraction of the electron beam irradiation is the shape
of the depth dose curve.
Region of more or less uniform dose followed by a rapid
drop off of dose offers a distinct clinical advantage over the
conventional x-ray modalities.
The depth in centimetres at which electrons deliver a dose
to the 80% to 90% isodose level.
Most useful treatment depth, or therapeutic range , of
electrons is given by the depth of the 90% depth dose.
Electron beam characteristics

10. Electron beam characteristics (Depth
distribution in
water)
• Rapid rise to 100%
• Region of uniform
dose (proximal
90% to distal 90%)
• Rapid dose fall-off
• High surface dose
• Clinically useful
range 5-6 cm
depth
Because the dose decreases abruptly beyond the 90% dose level,
the treatment depth and the required electron energy must be
chosen very carefully

11. 11
Electron Beams Characteristics
Central axis depth dose curves
6
18
Modest skin sparing
↑energy ↑skin dose
Relatively
uniform dose
Rapid dose drop-off
for low energy
electron beams, but
disappears for high-
energy electron
beams
Bremsstrahlung
x-ray
contamination
The choice of beam energy is much more critical for electrons than for photons.
R80(cm) ~ E(MeV)/2.8
R90(cm) ~ E(MeV)/3.2
increases
with
energy
dmax increases with energy for low-
energy electrons, ~ 2.5cm for
high-energy electrons (12-20 MeV)

12. 12
Characteristics of Clinical Electron
Beams
Central axis depth dose curves – buildup region
Lower energy electrons
scatter more and through
larger angles, causing more
rapid buildup, thus, the
difference between the
surface dose and maximum
dose is larger.
Higher energy electrons
scatter less and through
smaller angles, causing less
rapid buildup (in the extreme
case, if there is no scatter,
there will be no buildup).

13. Absorbed dose (also known as total ionizing dose, TID) is a
measure of the energy deposited in a medium by ionizing
radiation per unit mass, which may be measured as joules per
kilogram when it is represented by the equivalent SI
unit, gray (Gy).
The absorbed dose from a given level of incident radiation
depends on the absorbing medium. For instance, a soft X-ray
beam may deposit four times more dose in bone than in air, or
none at all in a vacuum.
Absorbed dose

14. 14
Determination of Absorbed Dose
Absorbed dose can be measured with:
ionozation chamber
calorimetry
Fricke dosimetry
Relative dose can be measured with:
Film: energy independence for electron beam, TLD
Diode : often used for electron beam measurement.

15. ISODOSE
CURVE

16. Beams of ionising radiation have characteristic
processes of energy deposition, hence the
Expected dose distribution can be estimated.
In order to represent volumetric and planar
variations in absorbed dose, distributions are
depicted by means of ISODOSE CURVES .

17. PDD(Percentage depth dose)
The quantity percentage depth
dose may be defined as the
quotient,
expressed as a percentage, of
the absorbed dose at any
depth 'd‘ to the absorbed dose
at a fixed reference depth 'd0'
,along the central axis of the
beam.

18. ISODOSE CURVES
DEFINITION:
Isodose curves are the lines joining the points of
equal Percentage Depth Dose (PDD). The curves are
usually drawn at regular intervals of absorbed dose and
expressed as a percentage of the dose at a reference
point.
ISODOSE CHARTS : It consists of a family of isodose curves.
The depth dose values of the curves are
normalized:
1) At the point of maximum dose on the central axis (Dmax)
2) At a fixed distance along the central axis in the irradiated
medium (SAD).

19. Measurement of isodose curves
1. Ion Chambers
2. Solid state detectors
3. Radiographic Films
4. Computer driven devices
Ion chamber is the most reliable method,
because of its relatively flat energy response
and precision

20. Most useful treatment depth , therapeutic range of
electrons is given by the depth of 90% of the isodose
curves……….
The PDD increases as the energy
increases.
However unlike photon beams ,
the percent of surface
dose for electron beam increases
with energy

21. 21
Electron Beams
For low energy electron
beams, isodose curves
bulging out for all dose
levels
Isodose curves

22. 22
Electron beam cont…
Isodose
curves
But bulge out for low dose levels
For high energy electron
beams, isodose curves
constrict for high dose levels

23. The term penumbra generally defines the region at the
edge of the radiation beam over which the dose rate
changes rapidly as a function of distance from the
beam central axis.
Penumbra
The physical penumbra of
an electron beam may be
defined as the distance
between two specified
isodose
curves at a specified
depth in phantom.

24. Treatment planning
in
Electron beam

25. As with photon beam treatments, the first step in the
initiation of electron therapy 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 margins to be treated before simulation and
placement of the electron fields is initiated.
Target definition

26. Most electron beam treatments are planned for a single field
technique.
For a relatively flat and homogeneous block of tissue, dose
distribution can be found using appropriate isodose chart.
Treatment planning is a exception rather
than a rule.
Surface areas are seldon flat, and in many cases
inhomogeneous such a bone,lung and air cavities, which
causes dosimetric calculations a little bid complex.

27. Treatment planning...........
A.Choice of Energy and field size
B.Correction for air gap and beam
obliquity
C.Tissue inhomogenicites
D.Use of bolus and absorbers
E.Problems of adjacent fields

28. Treatment planning...........
Choice of Energy and field size
The energy beam is directed in general by
The depth of the target volume.
Minimum target dose required
Cinically acceptable dose to a critical organ
The electron energy for treatment should be selected such
that the depth of the 90% isodose line covers the distal or
deepest portion of the region to be treated in addition to an
approximate 5-mm additional depth beyond the treatment
region

29. Correction for air gap and beam obliquity
Treatment planning...........
In electron beam therapy, there is a frequent problem
of the treatment of the cone end not being parallel to the
skin surface.
Uneven air gaps as a result of curved patient surfaces
are often present in clinical use of electron beam
therapy.
Inverse square law corrections can be made to the
dose distribution to account for the sloping surface

30. The inverse square correction alone does not
account for changes in side scatter as a result of
beam obliquity which:
 Increases side scatter at the depth of
maximum dose, Zmax
Shifts zmax toward the surface
Decreases the therapeutic depths R90 and R80.

31. Tissue inhomogenicity
Treatment planning...........
Electron beam dose distribution can be significantly altered
in presence of tissue heterogeneity such as bone, lung
and air cavities.
It is difficult to determine dose distribution in presence of
such conditions.
The simplest correction for a tissue inhomogeneity
involves the scaling of the in homogeneity thickness by
its electron density relative to that of water and the
determination of the coefficient of equivalent thickness
(CET)

32. Use of bolus and absorbers
Treatment planning...........
Bolus is often used in electron beam therapy
Flatten out an irregular surface
Reduce the penetration of electron in parts of the field
Increase in the surface dose
To shorten the range of a given electron beam in the
patient
Bolus made of tissue equivalent material, such as
wax

33. The use of computed tomography (CT) for treatment
planning enables accurate determination of tumour
shape and patient contour.

34. Occasionally, the need arises to abut electron fields.
When abutting two electron fields, it is important to
take into consideration the dosimetric characteristics
of electron beams at depth in the patient.
The large penumbra and bulging isodose lines
produce hot spots and cold spots inside the target
volume
CLINICAL CONSIDERATIONS
In general, it is best to avoid using adjacent electron
fields

35. Electron arc therapy
Electron arc therapy is a special radiotherapeutic
treatment technique in which a rotational electron beam
is used to treat superficial tumour volumes that follow
curved surfaces.
While its usefulness in treatment of certain large
superficial tumours is well recognized, the technique is
not widely used because it is relatively complicated,and
its physical characteristics are poorly
understood

36. Two approaches to electron arc therapy have been
developed:
• Electron pseudo-arc based on a series of overlapping
stationary electron fields.
• Continuous electron arc using a continuous rotating
electron beam
The calculation of dose distributions in electron arc
therapy is a complicated procedure that generally cannot
be performed reliably with the algorithms used for
standard electron beam treatment planning

37. Total Skin Irradiation
2-9 MeV electron beams are useful for treating
superficial lesions covering a large areas of the body
(e.g. mycosis fungoides)

38. The clinically useful energy range of electrons is 6 to 20 MeV, usefull
for treating superficial skin tumours of less than 6 cm
Energy of electron beam is specified by the most probable energy at
the surface.
Depth dose distribution can be determined by ion chamber, diodes and
film
Field shaping can be done with lead or cerrobend cutouts.
Electron arc threapy is feasible for tumors along curved surface also
with proper shielding.
Conclusion...