basic introduction of electron beam radiotherapy for M.D. radiation oncolgy students

1. By ANKITA PANDEY

2. Electron beam RT
External beam radiotherapy
Electrons are used to treat tumors
Superficial tumors

3.  Megavoltage electron beams are used
 Clinically useful energies are between 6 to 20
MeV

4.  Delivers uniform dose at specific depth
 Rapid dose fall off
 Better deep normal tissue sparing
 Effective treatment for superficial tumors

5.  Treating various superficial diseases ( within
6cm)
 Example
1. Cancer of skin regions or total skin ( eg
mycosis fungoids)
2. Melanoma
3. Lymphoma
4. Nodal boost
5. Breast cancer
6. Total body irradiation

6.  Electrons interact with matter by coulombs
force interaction
 The various process of interactions are
1. Inelastic collision with atomic electrons
2. Inelastic collision with nuclei
3. Elastic collision with atomic electrons
4. Elastic collision with nuclei

7.  Some energy is lost
 Excitation
 ionization

9.  Kinetic energy is not lost although it may be
redistributed among particles emerging from
collision

10. When beam of electrons passes through a medium
Scattering scattering power
varies as square of
atomic no. And
Due to coulomb force inversely as square
of interaction between of K.E.
incident electrons
and nuclei

11.  Clinical implication
 For this reason high atomic number materials
are used in the construction of scattering foils
used for the production of clinical electron
beams in the linac

12.  Central axis depth dose
 Isodose curves
 Field flatness and symmetry
 Field size dependence
 Field equivalence
 Electron source
 X ray contamination

13.  Depth dose curve
 Rapid rise to 100%
 Rapid dose fall off
 High surface dose
 Clinically useful range 5-6cm

15.  As dose decreases rapidly beyond 90% dose
level, the treatment depth and required
electron energy must be chosen carefully
 Most useful treatment depth, therapeutic
range of electrons is given by the depth of 90%
of isodose curves
 The PDD increases as the energy increases
 However, percent of surface dose increases
with energy

16.  R90 = E/3.2
 R80 = E/2.8

17.  Defined as absorbed dose at any depth ‘d ‘
to the absorbed dose at fixed reference
depth along central axis of beam expressed
in percentage

19.  Isodose curves are the lines joining the
points of equal PDD
 The curves are usually drawn at regular
intervals of absorbed dose and expressed as
percentage of dose at a reference point.

20. Lateral
constrictio
n
bulging

21.  As an electron beam penetrates a medium,
the beam expands rapidly below the surface,
due to scattering
 Individual spread of the isodose curves varies
depending on the isodose level, energy of
the beam, field size, and beam collimation

22.  Uniformity of electron beam is usually
specified in a plane perpendicular to the
beam axis and at a fixed depth
uniformity index
ratio of the area where dose exceeds 90% of
its value at the central axis to the geometric
beam cross sectional area at the phantom
surface

23.  Because of the presence of low energy
electrons in the beam, the flatness changes
significantly with depth
 Beam symmetry compares a lateral dose
profile on one side of central axis to that on
the other

24.  The output and central axis depth dose are
field size dependent
 Dose increases with the field size because of
increased scatter from the collimator

25. PENCIL ELECTRON
BEAM
Vacuum window
diverge
Virtual source point

26.  At the end of the electron range
 Contributed by the bremsstrahlung
interaction of the electrons with the
collimator system and the body tissues
 X ray contamination is least with the
scanning type of accelerators because
scattering foils are not used

27. Electron energy X ray contamination dose
6 to 12 MeV 0.5 to 1%
12 to 15 MeV 1 to 2%
15 to 20 MeV 2 to 5%

28.  Single field technique
 For flat surface and homogenous tissue, dose
distribution can be found by isodose chart
 Treating area is irregular and inhomogenous

29.  Choice of energy and field size
 Correction of air gaps and beam obliquity
 Tissue inhomogeneity
 Use of bolus
 Problems of adjacent feilds

30.  Energy of beam is decided according to
1. Depth of target volume
2. Minimum target dose required
3. Dose to OARs
If there is no risk of normal organ
overdosing then beam energy is selected so
that entire tumor volume is covered by 90%
to 95% isodose curve

31.  Choice of field size is based on isodose
coverage of PTV
 It has been seen that there is lateral
constriction of the 80% isodose curve at
energies above 7MeV
 Thus larger field size should be selected to
cover PTV adequately

32.  Uneven air gaps as a result of patient
surfaces are often present
 Inverse square law corrections can be made
to the dose distribution to account for the
sloping surface

33.  Electron beam dose distribution can be
significantly altered in the presence of tissue
heterogeneity such as bone, lung and air
cavities
 It is difficult to determine dose distribution
in presence of such conditions
 Coefficient of equivalent thickness (CET)

34. Used to
1. Flatten out an irregular surface
2. Reduce the penetration of electrons in
parts of the field
3. Increase the surface dose

35.  Materials used as bolus
1. Paraffin wax
2. Polyestyrene
3. Lucite
4. Superstuff
5. superflab

36.  Special technique in which a rotational
electron beam is used to treat superficial
tumor volumes that follow curved surfaces
 Not widely used as
relatively complicated
physical characteristics are poorly
understood

37.  Electron energy 2 to 9MeV
 Mycosis fungoids and other cutaneous
lymphomas
 Rapid fall off in dose beyond shallow depth
 Skin lesions extending to 1cm depth can be
effectively treated without exceeding bone
marrow tolerance

38.  Translational techniques
 Large field technique

41.  Useful range of electrons are 6 to 20 MeV
 Electrons interact with matter by elastic or
inelastic collision
 Energy of electron is specified by most
probable energy at the surface
 Modest skin sparing effect
 Percent surface dose increases with
increasing energy
 Electron arc therapy is feasible for tumors
along curved surfaces