Updates on Electron Beam Therapy I) Introduction II) Central Axis Depth dose distribution III) Dosimetric parametrics of electron beam IV) Clinical Considerations of Electron beam therapy

2. Out lines of presentation
➢Introduction
➢Central axis depth dose distributions in water
➢Dosimetric parameters of electron beam
➢Clinical consideration of electron beam
therapy

3. Introduction
• Megavoltage photon based RT for shallow tumor
is complicated by the buildup and radiation
transport properties of photon beams.
• These beams are capable of treating both shallow
& deep tumors but with distal normal tissue
exposure.
• Megavoltage electron beams have the property
of a finite range & do not deliver significant
doses to distal depths.

4. Introduction
• Even the strongest proponents of photon or
proton therapy acknowledge that electron
therapy is necessary to complete RT program.
• In fact, G.H. Fletcher had gone as far as saying,
“There is no alternative treatment to electron-
beam therapy.”

5. Introduction
• The widespread use of electron beam therapy
began in the 1970s when LINAC capable of
producing a range of electron energies
became common.

7. Introduction
• The intensity of a photon beam falls off gradually
as it penetrates matter.
• Electrons however, have a fairly well defined
range.
• The rate of energy loss depends on the electron
density of the medium.
• For electrons in the MeV energy range the energy
loss rate in water is about 2 MeV/cm
• For this reason the electrons will lose all of their
kinetic energy in a distance Rp ( c m )

8. Introduction
• .

9. Introduction
• Electrons exhibit less skin sparing than
photons.
• The percent depth dose at the surface is 80% -
90% as opposed to a typical value of about
30% for a photon beam.
• The relative surface dose increases with
energy as oppose of photons

10. Introduction
• There is a small residual “tail” to the PDD
curves which increases in amplitude with
increasing energy.
• The tail is due to bremsstrahlung x-ray
production by the electrons.
• The electrons interact with the scattering foil,
cone and collimator producing a small x-ray
contamination component.

11. • Central axis depth dose distributions
in water

12. Central axis depth dose distributions in water
• characteristics electron beam central axis PDD
curve :
– High surface dose (80 - 95%).
– Maximum dose occurs at a certain depth referred to
as the depth of dose maximum zmax.
- Beyond zmax the dose drops off rapidly to a small low
level dose called the bremsstrahlung depends on
electron beam energy and is typically:
➢<1% for 4 MeV electron beams.
➢ <2.5% for 10 MeV electron beams.
➢<4% for 20 MeV electron beams

14. Central axis depth dose distributions in
water

15. Electron scattering
• When a beam of electrons passes through a
medium, the electrons suffer multiple scattering
because of Coulomb force interactions.
• As a result, the electrons acquire velocity
components and displacements transverse to
their original direction of motion.

17. Electron interactions with absorbing medium
• Electrons traversing an absorber lose their kinetic energy
through ionization collisions and radiation collisions.
• The rate of energy loss per gram and per cm2 is called the
mass stopping power and it is a sum of two components:
- Mass collision stopping power
- Mass radiation stopping power
• The rate of energy loss for a therapy electron
beam in water & water-like tissues, averaged over
the electron’s range, is about 2MeV/cm

18. Electron Interactions
Inelastic collisions
1. atomic electrons (ionization & excitation)
2. nuclei (bremsstrahlung)
Elastic collisions
1. atomic electrons
2. nuclei

19. Characteristics of clinical electron beam
• Central depth dose
• The major attraction of the electron beam
irradiation is the shape of the depth dose curve,
especially in the energy range of 6 to 15 MeV.
• A 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.

20. Range concept
• The path length of a single electron is the total
distance travelled along its actual trajectory.
• The projected path range is the sum of
individual path lengths projected on to the
incident beam direction.

21. Range concept
• The maximum range Rmax is defined as the
depth at which extrapolation of the tail of the
central axis depth dose curve meets the
bremsstrahlung background
• It is the largest penetration depth of electrons
in the absorbing medium.

22. Range concept
• The practical range Rp is defined as the depth
at which the tangent plotted through the
steepest section of the electron depth dose
curve intersects with the extrapolation line of
bremsstrahlung
• The depth Rq is defined as the depth where
the tangent through the dose inflection point
intersects the maximum dose level,

23. Therapeutic range R90
• The depth of the 90% dose level (R90 (cm))
beyond zmax is defined as the therapeutic
range for electron beam therapy.
• This depth is approximately given by E/4 in Cm
of water, where E is the nominal energy in
MeV of the electron beam.
• R80 (cm), the depth that corresponds to the
80% PDD beyond zmax is approximated by
E/3 in Cm of water.

24. Electron arc therapy

25. Typical electron beam depth dose parameters for electron beam

26. Electron beams can be considered almost mono-energetic as
they leave the accelerator; however, as it passes through the
linac exit window, monitor chambers, collimators and air, the
electrons interact with these structures resulting in:
➢ broadening of the beam’s electron energy spectrum.
➢Bremsstrahlung production contributing to the
bremsstrahlung tail in the electron beam PDD distribution
Electron beam characteristics

27. On initial contact with the patient, electron beam has an incident
mean energy E0 that is lower than energy inside the accelerator.
Electron beam characteristics
The ratio of the dose at a given point on the central axis of an
electron beam to the maximum dose on the central axis
multiplied by 100 is the percentage depth dose (PDD).
The percentage depth dose is normally measured for the
nominal treatment distance and depends on field size and
electron beam energy.

28. Electron beam characteristics
• 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

29. PDD
• Similar to PDDs for photon beams, the PDDs
for electron beams, at a given source-surface
distance SSD, depend upon:
– Depth z in phantom (patient).
– Electron beam kinetic energy
EK(0) on phantom surface.
– Field size A on phantom
surface.

31. Buildup region
• The dose buildup in electron beams is less
pronounced than megavoltage photon beams
and results from the scattering interactions of
electrons.
• Upon entry into the medium electron paths
are approximately parallel.
• With depth their paths become more oblique
with regard to the original direction.

32. Buildup region
• The buildup region is the depth region b/n
the phantom surface & depth of dose
maximum zmax.
• The surface dose for megavoltage electron
beams is relatively large (75-95%) in contrast
to surface dose of photon beams( 10-25%).

33. • Dosimetric parameters of electron
beam

34. Characteristics of clinical electron beam
• Central depth dose
• The most useful treatment depth, or
therapeutic range, of electrons is given by the
depth of the 90% depth dose.
• For modern accelerators this depth is
approximately given by E/3.2 cm, where E is the
energy in MeV of the electron beam at the
surface.
• The depth of the 80% depth dose occurs
approximately at E/2.8 cm.

35. Characteristics of clinical electron beam
• Central depth dose
• The choice of beam energy is much more critical
for electrons than for photons.
• B/c the dose decreases abruptly beyond the 90%
dose level, the treatment depth and the
required electron energy must be chosen very
carefully.
• The guiding principle is that, when in doubt, use
higher electron energy to make sure that the
target volume is well within the specified
isodose curve.

36. Characteristics of clinical electron beam
• Central depth dose
• The skin-sparing effect with the clinical electron
beams is only modest or nonexistent.
• Unlike the photon beams, the percent surface
dose for electrons increases with energy.
• This effect can be explained by the nature of the
electron scatter.

37. • Clinical consideration of electron
beam therapy

38. Isodose curves
• Isodose curves are lines passing through
points of equal dose.
• As an electron beam penetrates a medium,
the beam expands rapidly below the surface,
due to scattering.
• However, the individual spread of the isodose
curves varies depending on the isodose level
& energy of the beam

39. Isodose curves
• A particular characteristic of electron beam
isodose curves is:
• Bulging of the low value curves (<20%) as a
direct result of the increase in electron
scattering angle with decreasing electron
energy.
• Lateral constriction at energies above 15 MeV,
at higher value isodose curves (>80%).

42. Electron applicators
• Photon beam collimators on the accelerator
are too far from the pt to be effective for
electron field shaping.
• After passing through the scattering foil, the
electrons scatter sufficiently with the other
components of the accelerator head & air to
create a clinically unacceptable penumbra.

43. Electron applicators
• Electron beam applicators or cones attached
to the treatment unit head are used to
collimate the beam.
• Several cones are provided, usually in square
field sizes ranging from 5 × 5 cm2 to 25 × 25
cm2.

46. Shielding and cut-outs
• For a more customized field shape, a lead or
metal alloy cut-out may be constructed and
placed on the applicator as close to the
patient as possible.
• As a rule of thumb, simply divide the practical
range Rp by 10 to obtain the approximate
thickness of lead required for shielding (<5%
transmission).

48. Field shaping
• Field shaping for electron beams is always
achieved with electron applicators (cones),
which may be used alone or in conjunction
with shielding blocks or special cut-outs.

49. Field Shaping

51. Internal shielding
• For treatments of the lip, buccal mucosa,
eyelids or ear lobes it may be advantageous to
use an internal shield .

52. Bolus
• Bolus, made of a tissue equivalent material
such as wax, is often used in electron beam
therapy for the following purposes:
➢To increase the surface dose;
➢To flatten out irregular surfaces;
➢To reduce the electron beam penetration in
some parts of the treatment field.

53. Bolus
• For very superficial lesions, the practical range
of even the lowest energy beam available
from a linac may be too large to provide
adequate healthy tissue sparing beyond the
tumour depth.
• To overcome this problem, a tissue equivalent
bolus material of specified thickness is used to
shorten the range of the beam in the patient.

54. Bolus
• Sharp surface irregularities, where the
electron beam may be incident tangentially,
give rise to a complex dose distribution with
hot and cold spots.
• Tapered bolus around the irregularity may be
used to smooth out the surface and reduce
the dose inhomogeneity.

56. Treatment planning
• Most electron beam treatments are planned for
a single-field technique.
• For a relatively flat and homogeneous slab of
soft tissue, the dose distribution can be found by
using the appropriate isodose chart.

57. Treatment planning
• The energy of beam is dictated by:
➢ Depth of the target volume
➢Minimum target dose required
➢Clinically acceptable dose to a critical organ
• In most cases, when there is no danger of
overdosing a critical structure beyond the target
volume, the beam energy may be set so that the
target volume lies entirely within the 90% to
95% isodose curve.

58. Treatment planning
• However, in the treatment of the breast (e.g.,
chest wall, after mastectomy)the energy is often
chosen so that the depth dose at the chest wall–
lung interface is about 80%

59. FIELD MATCHING
• Occasionally, situations arise in which adjacent fields
must be matched in order to avoid an over- or under-
dose.
• Situations in which such beam matching may be
required include:
➢ Providing varying penetration by using different energy
electron beams for adjoining areas
➢ Treating a larger area than standard applicators allow
➢ Treating an area adjacent to a previously-treated area
➢ Reducing the extreme effects of obliquity when
treating a curved surface

60. FIELD MATCHING
• In beam matching, the joining should be
positioned away from critical areas.
• Staggered joins (i.e. shifting joins at different
treatment fractions) may be considered if this
is practical.

61. FIELD MATCHING
A) Electron–electron Match:
• Both electron fields will bow into each other and cause
severe hot spots.
• Avoid if possible.
B) Electron-Photon Match:
-A skin gap may be used to limit this hot spot.
• Like all skin gaps, this results in a superficial cold spot.
-Gantry rotation may also be used to limit this hot spot.
• This also results in a superficial cold spot.

63. In homogeneity corrections
• The dose distribution from an electron beam
can be greatly affected by the presence of
tissue in homogeneities such as lung or bone.
• The dose within these in homogeneities is
difficult to calculate or measure.

64. In homogeneity corrections
• Inhomogeneities in composition and in shape
modify dose distributions in two ways,
I) The first is the effect on absorption and the
consequent shift in isodose lines
II) The second is due to scatter differences
between different materials

65. In homogeneity corrections

66. Electron Arc therapy
• The problems of dose inhomogeneity associated
with obliquity and beam matching are effectively
eliminated with the use of narrow-beam electron
arcing techniques.
• Electron arc therapy is a special RT technique in
which a rotational electron beam is used to treat
superficial tumour that follow curved surfaces.
• It is relatively complicated and its physical
characteristics are poorly understood.

67. Electron Arc therapy
• Two approaches to electron arc therapy have
been developed:
• Electron pseudo arc – simpler & is based on a
series of overlapping stationary electron fields
• continuous rotating electron beam.

69. Thank You!
• “There is no alternative treatment
to electron-beam therapy.”