2. The electron e is an elementary particle that
carries a negative charge
the electron was finally discovered by J. J.
Thomson in 1897 While studying so-called
cathode rays, which in fact are electron
beams, he discovered that these rays are
negatively charged particles, which he called
3. In 1924, the wave-particle dualism was
by de Broglie
All moving matter has wave properties with the
wavelength λ being related to the momentum p
λ= h / p = h / mv
this means that accelerated electrons act not
only as particles but as waves too.
4. An electron accelerated in an electric field V
gains an energy E = eV
which further corresponds to a kinetic energy
E KE = mv2/2
E = eV = mov2 / 2
5. Electron beam radiation is a special type of
radiotherapy that consists of very tiny
electrically charged particles
The electron beams most commonly available
in radiotherapy departments have energies
of between 4 MeV and 25 MeV as produced
by standard clinical linear accelerators,
although some microtrons provide higher
6. Electron beams have advantages for a variety of
clinical situations due to the characteristics of
their depth-dose curves
They deliver acceptably uniform doses to a
relatively well-defined region extending from the
surface to the therapeutic range , which can be
altered to fit the clinical situation by varying the
So the electron beams can be used for treating
superficial tumors (<5 cm deep) with a
characteristically sharp dropoff in dose beyond
7. The principal applications are
The treatment of skin and lip cancers
Chest wall irradiation for breast cancer
Administering boost dose to nodes
The treatment of head and neck cancers
8. Electron Interactions
As electrons travel through a medium, they
interact with atoms by a variety of processes
owing to coulomb force interactions.
Inelastic collisions with atomic electrons
(ionization and excitation)
Inelastic collisions with nuclei
Elastic collisions with atomic electrons
(e-e scattering )
Elastic collisions with nuclei
9. • Inelastic collisions, some of the kinetic energy
is lost as it is used in producing ionization or
converted to other forms of energy such as
photon energy and excitation energy.
• Elastic collisions, kinetic energy is not lost,
although it may be redistributed among the
particles emerging from the collision
10. 1 . inelastic collisions with atomic electrons
• Columb force of interaction between incident
electron and orbital electron of an absorber
results in ionization and excitation of atom.
• This results in a collisional energy losses and
this described by collision stopping power
11. 2 . inelastic collisions with nuclei
• Columb interaction between electron and
nuclei of the absorber atom results in an
electron scattering and energy loss of electron
by bremsstrahlung x-rays
• This type of energy loss characterized by
radiative stopping power
12. Stopping Power
The total KE loss of the charged particle in the
absorber per unit path length defined as the
linear stopping power.
S = dE/dl
(Unit is MeV/cm)
13. The rate of energy loss depends on the electron
density of the medium
The rate of energy loss per gram per centimeter
squared is called the mass stopping power
14. • mass stopping power, is greater for
low-atomic-number (Z) materials than for high-Z
There are two reasons for this: First,
high-Z materials have fewer electrons per
gram than low-Z materials
high-Z materials have more tightly bound
electrons, which are not as available for this type of
15. • The integral for stopping power can alternatively be
cast in terms of the impact parameter ‘b’ and then
integrating out to b = ∞ yields an infinite stopping
power because of the large number of soft
collisions at large distances.
• Bohr’s conceived of quantized energy levels for the
atomic electrons, in terms of the time of collision, t,
and the natural frequency of the atomic electron, n,
• such that the displaced electron, oscillate about its
natural equilibrium position with a period of 1/n.
16. • If the collisions are such that ‘t’ is short, then
the electron would behave as though it were
free and accept the impulse (corresponding to
• on the other hand, the collision time was
relatively long, then the electron acts as though
bound; its orbit is distorted or deformed by the
passage of the charged particle, but no net
energy transfer takes place.
17. • the energy loss rate first decreases and then increases with
increase in electron energy
• minimum occurring at about 1 MeV.
Above 1 MeV, the variation with energy is very gradual.
• The energy loss rate of electrons of energy 1 MeV and above
in water is roughly 2 MeV/cm.
18. 3.Electron Scattering
• The electron has small mass , so when a beam
of electrons passes through a medium it suffer
multiple scatter through coulomb interaction
between the incident electron and
predominantly the nuclei of the medium.
• There is no significant loss of electron energy
but the trajectory is deflected from the original
19. • The Coulomb force F is defined as:
F = Q1Q2 / 4π εo r2
(r- distance between the charges Q1 and Q2; εo : dielectric constant).
• The closer the electron comes to the nucleus, i.e.
the smaller r, the larger is F and consequently the
20. • Because of its dependence on the charge, the
force F with which an atom attracts an
electron is stronger for atoms containing more
positives charges, i.e. more protons.
• Thus, the Coulomb force increases with
increasing atomic number Z of the respective
21. • The scattering power of electrons varies
approximately as the square of the atomic number
and inversely as the square of the kinetic energy.
• For this reason high atomic number materials are
used in the construction of scattering foils used for
the production of clinical electron beams in a linac.
22. CENTRAL AXIS DEPTH DOSE
DISTRIBUTIONS IN WATER
• The general shape of the central axis depth dose
curve for electron beams differs from that of
23. • The electron beam central axis depth dose curve
exhibits a high surface dose (compared with
megavoltage photon beams), and the dose then builds
up to a maximum at a certain depth referred to as the
electron beam depth of dose maximum zmax.
• Beyond zmax the dose
drops off rapidly and
levels off at a small low
level dose component
referred to as the
24. • These features offer a distinct clinical advantage
over the conventional X ray modalities in the
treatment of superficial tumors
• As the electron beam passes through the
accelerator exit window, scattering foils, monitor
chambers, collimators and air, the electrons
interact with these structures,
• In Resulting , Bremsstrahlung production ,
contributing to the bremsstrahlung tail in the
electron beam PDD distribution
25. • An electron beam is almost monoenergetic before
striking the accelerator window,
• The random energy degradation that the electrons
suffer as they pass through the exit window,
scattering foil, monitor chambers, air, and other
• This results in the beam taking on a spectrum of
energies at the phantom surface.
• Further degradation and spread of beam energy
take place with depth in the phantom
27. Energy Specification and Measurement
• An electron beam is usually characterized by
the energy at the body surface
• Range measurements used to determine this
28. Most Probable Energy (Ep)
In clinical practice, an electron energy is specified
by the most probable energy at the surface (Ep).
This is the kinetic energy possessed by most of
the electrons incident at the surface
f.S < 12 × 12 cm^2 for energies up to 10 MeV
> 20 × 20 cm^2 for higher energies.
(Ep)0 = C1 + C2Rp + C3Rp
29. • Range measurements are usually made using
the depth ionization curve
• Each point on the depth ionization curve should
be corrected for beam divergence before the
range is determined.
• The correction factor is
f - effective SSD
z - is the depth
30. RANGE CONCEPT
Range is the distance travelled by a particle in
the stopping medium.
One can estimate the damage to the medium
due to the ionizing radiation.
This is more important especially in the fields
of Radiotherapy, Surface Analysis and
31. Rp - PRACTICAL RANGE
• The practical range Rp (cm or g/cm2) 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 the
background due to
32. The practical range, Rp, represents those
electrons that have travelled through the
material with the minimum of scatter
deviations from the original direction
producing a straight or almost straight path.
It can be estimated in cm of water or soft
tissue as roughly half the beam energy in MeV
33. Rmax - MAXIUM RANGE
The maximum range Rmax (cm or g/cm2) 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
34. BEAM MEAN ENERGY
• The Mean Energy, E0 , of the electron beam,
used in dosimetry, is given by
E0 = C4 . R50
• where C4 = 2.4MeV/cm
• R50- is the depth of 50% dose.
35. Energy at Depth z
6 6.49 5.94
9 9.34 8.78
12 12.25 11.64
16 15.54 14.76
20 20.54 19.19
36. PDD - PROPERTIES
The typical features of an electron depth dose chart are ….,
• A high surface dose relative to photon beams
• A broad ‘effective dose’ region
• A linear fall off in dose at depth
• A bremsstrahlung tail due to generation of
photons from inelastic 'collisions' with nuclei
37. • Electron beam depth-dose curve illustrates the
relatively uniform dose region from surface to
• Therapeutic range, typically taken to be the depth
of the distal 90% or 85% dose depending on local
practice, and the rapidly falling part of the curve
• The therapeutic range can be roughly estimated in
cm of water or soft tissue as one-third of the beam
energy in MeV
38. surface dose
• The surface dose (Ds), conventionally stated at 0.5
mm depth, Generally lies between around
75% - 80% for lower-energy beams (4- 6MeV)
90% - 100% for higher-energy beams (20-25 MeV).
• Unlike the photon beams, the percent surface dose for
electrons increases with energy.
• The skin-sparing effect with the clinical electron
beams is only modest or nonexistent.
39. • This effect can be explained by the nature of
the electron scatter. At the lower energies, the
electrons are scattered more easily and
through larger angles
• The rise in relative dose from the surface is
due to the increasing obliquity of the electron
paths ( as a result of scatter as the electrons
penetrate the material )
40. • This , increases the mean path length travelled
for fixed increments of depth into the material
and increases the electron fluence with depth
• Thus, the energy deposited in successive
layers of the material increases
41. • This causes the dose to build up more rapidly and
over a shorter distance
• I.e. For lower energy electrons, lateral scattering
happens at shorter distance after
they enter the tissue.
This leads to a relatively
rapid loss of energy, with
a significant 'peak' of
energy loss at zmax relative
to the surface dose.
42. • Higher energy electron beams tend to undergo
minimal scattering near the surface and continue
onwards, losing their energy over a greater
• This leads to significantly broader region of dose
distribution, and zmax is not significantly greater
than the surface dose.
• The final outcome of these interactions is that
high energy electrons have a high surface dose
relative to low energy electrons
44. • The surface dose and 100% at R100 is governed by
the scatter properties of the beam
• Therefore it depends not only on beam energy, but
also on the details of the accelerator head design
• because this determines the angular distribution of
the incident electrons.
Because of differences in beam generation, beam
bending, and collimation, the depth dose
distribution and the surface dose can be quite
different for different machines
45. Point Of Maximum Dose (Dmax)
• The depth of Dmax does not follow a linear
relationship with energy but it covers a broad
• Its value in cm of water
for a broad beam may be
approximated by 0.46 E0.67
E - most probable beam
energy at the surface in MeV.
• zmax usually occurs at a depth of E/4
46. • In general, dm starts at shallow depths and increases as
energy increases because the electron range increases
• As energy increases, the region around Dm becomes
• In this case dm is defined at a selected point.
(more relevant to define R100 as the mid-depth between
distal and proximal 99% or 98%.)
47. • Like photon beams, the depth of maximum dose in
electron beams zmax does not follow a specific trend
with electron beam energy.
• It is a result of the machine design and accessories
The most useful treatment depth, or therapeutic range,
of electrons is given by the depth of the 90% depth
It is the depth of 90% dose
clinically relevant point
beyond which dose is not
49. • For modern accelerators this depth is approximately
given by E/3.2 cm,
E - is the most probable energy in MeV of the
electron beam at the surface.
• The depth of the 80% depth dose occurs
approximately at E/2.8 cm.
• It is also a frequently used parameter for defining the
• It can be approximated by
E/3 in cms of water.
• Depth of the 50% dose beyond dmax.
• It is the depth at which the ionization curve falls to
50% of its maximum
• The depth R50 is the
beam quality index in
electron beam dosimetry
as specified in
IAEA TRS 398.
52. Dose fall-off
• Scattering and continuous energy loss by electrons
are the two processes responsible for the sharp drop-
off of dose at depths beyond Dmax.
• steep as energy increases,
significantly so for beams
having initial energies above
approximately 20 MeV,
• It depending on machine
54. • It is typically less than 1% for 4 MeV and
less than 4% for 20 MeV electron beams for
an accelerator with dual scattering foils
• The bremsstrahlung x rays are created by
collisions of the electrons with parts of the
machine and with the patient.
• The amount of bremsstrahlung
contamination increases with energy
because the probability for radiative
interaction increases with electron energy.
55. • It prevents the dose from going to zero at
• Structures lying beyond the electron range still
receive dose from bremsstrahlung x rays.
56. 2, 3, 4, 5 rule
• It is an alternative general rule for estimating the
depth of significant doses on the central axis of
• It is used to find the central-axis depth (in mm) of
the 100%, 90% and 50% doses and the practical
range by multiplying the incident beam energy (in
MeV) by 2, 3, 4 and 5, respectively.
• Although it is only an approximate rule, so a
rough estimate of this value
57. For 6 MeV…..
Dmax – 6 x 2 = 12 mm or 1.2 cm
R90 – 6 x 3 = 18 mm or 1.8 cm
R50 – 6 x 4 = 24 mm or 2.4 cm
Rp – 6 x 5 = 30 mm or 3 cm
58. VARIATION WITH BEAM ENERGY
• The relative surface dose increases.
• The depth of dose maximum increases
This characteristic can
vary with machine and
does not necessarily vary
in a monotonic way with
59. • The penetration increases, as reflected in the
values of therapeutic range, R50, and practical
• The steepness of the falling part of the curve
is roughly constant at the lower energies, but
decreases at the higher energies.
• The x-ray tail increases.
60. VARIATION WITH FIELD SIZE
• The field is large enough to produce scatter
equilibrium at the central axis.
• Below these dimensions, more electrons are
scattered away from the central axis, reducing
the fluence and the dose deposited.
• This imbalance results in the high-value depth
doses moving progressively towards the surface
as the field size reduces, with an increase in the
surface dose relative to maximum dose
61. • The maximum penetration of electrons (electron
range ) stays the same.
62. • The steepness of the depth–dose curve is
progressively reduced , Thus the practical range
is hardly changed.
• At medium and large field sizes, central-axis
depth doses do not vary significantly in their
64. VARIATION WITH FIELD SHAPE
• Where the field shape has all dimensions greater
than the scatter equilibrium value, the
depth–dose characteristics will be the same as for
medium to large square fields.
• Where any dimension is smaller than the critical
value, the depth dose will be modified
• The exact depth dose will depend on the shape
and the sizes involved
65. Hogstrom et al. 1981 suggested …….
for rectangular fields of sides a x b ,
PDD at a given depth can be estimated from square
field information, using
• The concept of a single equivalent field has limited
66. VARIATION WITH SSD
• In general, electron treatments are carried out at
extended SSDs because of the applicator
• To deal with this situation, a virtual source may be
• The v s p defines the effective SSD that can be used in
conjunction with an inverse square law correction for
that beam energy and applicator size
• After the virtual source position is determined,
corrections can be made to the depth–dose curve
67. • In general, because the penetration of electron beams is
not very large, an ISL correction to depth–dose curves
is not very significant, particularly for changes in SSD
of up to 10 cm
• Changes might be expected
close to the surface and
possibly extending to around
the depth of therapeutic range
68. • Relative doses close to the surface decrease,
but the depth of therapeutic range increases
• The main problem with increased SSD is that
of flatness and penumbra
69. VARIATION WITH OBLIQUE INCIDENCE
• Oblique incidence has an effect on the depth–
dose distribution due to changes in the effective
penetration of the electrons and changes in
• As the angle between the electron-beam central
axis and a perpendicular to the surface
increases, the depth–dose curve pulls towards
the surface in the region of the therapeutic
72. • The dose at the dm increases and becomes
greater than the zero angle maximum , when
the angle becomes steep
• This is due to scatter from upstream excess of
• Conversely, scatter from downstream missing
tissue penetrates to depths larger than the
practical range, increasing the residual dose.
73. ISODOSE CURVES
• Isodose curves are lines passing through points of
• Isodse chart , family of Isodose curves, usually
drawn at equal increment of PDD , representing
the variation in dose as a function of depth and
transverse distance from CAX
• Depth dose values of the curve are normalized
either at dmax or at a fixed distance along CAX
75. These distributions illustrate
the typical features….
• The relatively uniform dose
region between the surface
and the therapeutic range
• The rapid fall-off with depth
• At the surface, the penumbra is narrow, as the
field is sharply defined by the applicator close to
76. • Since the electron beam energy is being constantly
degraded as it penetrates through the patient there is an
increasing amount of laterally scattered electrons which
results in an increasing penumbra with depth
77. • A particular characteristic of electron beam
Isodose curves is the bulging of the low value
curves (<20%) as a direct result of the increase
in electron scattering angle with decreasing
• At energies above 15 MeV, electron beams
exhibit a lateral constriction of the higher
value Isodose curves (>80%).
78. Field Flatness and Symmetry
• Uniformity of the electron beam is usually
specified in a plane perpendicular to the beam
axis and at a fixed depth.
• The ICRU specifies beam flatness in terms of a
• Ratio of the area where the dose exceeds 90% of
its value at the central axis at a reference plane
and at a reference depth to the geometric beam
cross-sectional area at the phantom surface
79. • In addition, the dose at any arbitrary point in the
reference plane should not exceed 103% of the central
80. • Because of the presence of lower-energy electrons in
the beam, the flatness changes significantly with
• Therefore, it has been recommended that the
uniformity index be defined at the depth of half the
• Furthermore, it is defined as the ratio of the areas
inside the 90% and 50% isodose lines at this depth.
• A uniformity index of 0.70 or higher is acceptable with
field sizes larger than 10 X 10 cm2.
• The peak value in this plane should be less than 103%.
81. • The AAPM recommends that the flatness of an
electron beam be specified in a reference plane
perpendicular to the central axis, at the depth of the
95% isodose beyond the depth of dose maximum.
• The variation in dose relative to the dose at central
axis should not exceed ±5% (optimally to be within
±3%) over an area confined within lines 2 cm inside
the geometric edge of fields equal to or larger than 10
× 10 cm
82. Beam symmetry compares a dose profile
on one side of the central axis to that on the
• The cross-beam profile in the reference plane
should not differ more than 2% at any pair of
points located symmetrically on opposite sides
of the central axis
• The term penumbra generally defines the region at
the edge of a radiation beam over which the dose rate
changes rapidly as a function of distance from the
beam central axis
• The penumbra for electron beams is defined in terms
of the distance between two isodose values on a beam
profile at the depth of maximum dose (or at the
standard measurement depth) Or , indirectly in terms
of distances between specified isodoses and the
geometric field edge under stated conditions as
84. • generally the 20%–80% width is expected to be 10
mm to12 mm for electron beams below 10 MeV, and
8 mm to10 mm for electron beams between 10 MeV
and 20 MeV.
• These values apply for applicators with the final
collimation stage at 5 cm or less from the skin.
• for greater separation between the applicator
and the skin the penumbra will increase.
85. Isodose curves and Beam Collimation
• Acceptable field flatness and symmetry are
obtained with a proper design of beam scatterers
and beam-defining collimators.
• Usually use one or more
scattering foils in head
of machine , made up
of lead, to widen the beam
as well as give a uniform
dose distribution across
the treatment field.
86. • The beam-defining collimators are designed to
provide a variety of field sizes and to maintain
or improve the flatness of the beam.
• Basically, all collimators provide a primary
collimation close to the source that defines
the maximum field size and a secondary
collimation close to the patient to define the
87. • In the electron therapy mode, the x-ray
collimator jaws are usually opened to a size
larger than the cone or the applicator
• Because the x-ray jaws give rise to extensive
electron scatter, they are interlocked with the
individual cones to open automatically to a
fixed predetermined size.
88. Field Size Dependence
• The output and the central axis depth dose distribution
are field size dependent.
• The dose increases with field size because of the
increased scatter from the collimator and the phantom
89. • electron collimators provide a fixed jaw
opening, and the treatment field size is varied
by various-size applicators
• Such an arrangement minimizes the variation
of collimator scatter, and therefore, the
output variation with field size is kept
90. • If the jaw, were allowed to change with the
treatment field, the output would vary too
widely with field size, especially for lower-
• where This effect is shown
cone size is held fixed while
the x-ray jaws are varied
91. • As the field size reduces, the central uniform portion of
the isodose distribution reduces in width.
• minimum field radius for the
establishment of lateral scatter
equilibrium at all depths on central
axis is given by the following
92. VARIATION OF ISODOSES WITH SSD
• larger gaps result in greater penumbra widths, as
head-scattered electrons are allowed to diverge more
before being incident on the patient or phantom
• Hence, increasing SSD does not generally produce
wider high-dose regions; instead, these may remain
approximately constant or even be reduced in width.
• Beam flatness may also be altered by increased SSD
94. VARIATION OF ISODOSES WITH OBLIQUE INCIDENCE
• Isodose lines generally tend to roughly follow surface shape
up to incident angles of around 30deg
• As obliquity increases above this, the electron paths become
more tangential, air gaps increase and scatter changes become
• The isodoses move increasingly closer to the surface and the
penumbral region becomes increasingly wide
• At larger angles, scatter effects, as discussed for oblique
incidence depth doses, can produce increased dose hot-spots
96. Treatment Planning
Electron beam is useful because it delivers a
reasonably uniform dose from the surface to a
specific depth, after which dose falls off rapidly,
eventually to a near-zero value
A checklist needs to be followed for treatment
• Choice of energy
• Collimation and shaping and Bolus techniques
• Field Abutment
• Calculation of monitor units
• The dose specification for treatment is commonly given
at a depth that lies at, or beyond, the distal margin of the
disease, and the energy chosen for the treatment depends
on the depth of the lesion to be treated.
• To maximize healthy tissue sparing beyond the tumor,
while at the same time providing relatively
homogeneous target coverage, treatments are usually
prescribed , planning target volumes should be covered
by the 90% isodose (AAPM ) , 95% (ICRU ) , 85%
98. • Pragmatically, in electron planning, it is common
practice to aim to cover the planning target volume
with the 90% isodose
• In some instances lower-value isodoses may be
chosen, as for example, in chest wall irradiation, in
which the 80% isodose may be chosen to lie at the
lung surface in order to reduce penetration into the
lung and hence overall lung dose.
• If the treatment dose is specified at either R80 or R90,
the skin dose will often be higher than the
99. • The maximum dose to the patient could be up to
20% higher than the prescribed dose.
• The maximum dose should therefore always be
reported for electron beam therapy
• The choice of beam energy is much more critical for
electrons than for photons.
• Because the dose decreases abruptly beyond the 90%
dose level, the treatment depth and the required
electron energy must be chosen very carefully.
100. • The guiding principle is that, when in doubt, use a
higher electron energy to make sure that the target
volume is well within the specified isodose curve.
101. • For the Selection of energy in planning ,
a general rule is the following
Beam energy should be selected to ensure that:
• R90 > maximum depth of PTV
• Rp < minimum depth of critical structures
Rules of thumb (water)
1. Ep,o(MeV) ≈ 3.3 x R90(cm)
2. Ep,o(MeV) ≈ 2.0 x Rp(cm)
Therefore, to estimate beam energy:
1. Ep,o(MeV) > 3.3 x maximum depth in cm of PTV
2. Ep,o(MeV) < 2.0 x minimum depth in cm of CS
102. • An example ....
consider irradiating the posterior cervical
nodes of the neck with electrons to spare the
spinal cord because its dose is already near
tolerance. If the maximum depth of the nodes
is 3 cm and the minimum depth of the spinal
cord is 6 cm, calculate the energy that should
be used to treat
Ep,o(MeV) > 3.3 x 3 = 9.9 MeV
Ep,o(MeV) < 2.0 x 6 = 12 MeV
103. then using equations discussed, indicate that
minimum energy to cover the PTV is 9.9 MeV
maximum energy that protects the spinal cord
is 12 MeV,
Hence, beam energies in the range of 10 to 12
MeV should be acceptable
104. Collimation and shaping
• Most accelerators have a set range of electron
applicators (or cones) that allow limited flexibility
in modifying field shape.
• For other sizes, or for irregular fields, shaping
can be achieved relatively simply using cut-outs
of lead or low-melting-point alloy attached to, or
close to, the end of the applicator or on the
105. • As the electron scatter is more significant , so
Electron beam applicators or cones are usually used
to collimate the beam, and are attached to the
treatment unit head such
that the electron field is
defined at distances as
small as 5 cm from the
• Applicators are made up
of low z material (Al, plastic)
to minimize the x-ray
106. • 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.
• An important consideration in electron beam
shielding is to make certain that the thickness is
appropriate to reduce the dose to an acceptable value
108. INTERNAL SHIELDING
• For certain treatments, such as treatments of the
lip, buccal mucosa, eyelids or ear lobes, it may be
advantageous to use an internal shield to protect
the normal structures beyond the target volume.
• Lead shielding may be used to reduce the
transmitted dose to an acceptable value
• Care must be taken to consider the dosimetric
effects of placing lead shielding directly on the
109. • However, the electron backscatter from lead enhances
the dose to the tissue near the shield
• To dissipate the effect of
a suitable thickness of
absorber such as bolus
may be placed between
the lead shield and the
preceding tissue surface
110. Example ......
• A buccal mucosa lesion is treated with a
9-MeV electron beam incident externally on
the cheek. Assuming cheek thickness,
including the lesion, to be 2 cm,
(a) the thickness of lead required to shield oral
structures beyond the cheek,
(b) the magnitude of electron backscatter
111. Practical range= E/2 = 4.5
Energy at a depth ( Ēz ) =
= Energy x (1 – depth/practical range)
= 9 X (1- 2 / 4.5 )
Energy at 2cm = 5Mev
Lead thickness= e/2 +1
= 5/2 +1 = 3.5mm
EBS = 1.56
Bolus, made of a tissue equivalent material such as
wax, is often used in electron beam therapy for the
To increase the surface dose;
To flatten out irregular surfaces;
To reduce the electron beam penetration In some parts
of the treatment field.
113. • 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
• Bolus may also be used to define more precisely the range
of the electron beam.
• The difference between the available electron beam
energies from a linac is usually no less than 3 or 4 MeV.
• If the lower energy is not penetrating enough and the next
available energy is too penetrating, bolus may be used with
the higher energy beam to fine tune the electron beam
114. • Bolus can also be used to shape isodose lines to
conform to tumour shapes.
• Sharp surface irregularities, where the electron beam
may be incident tangentially, give rise to a complex
dose distribution with hot and cold spots.
• bolus around the irregularity may be used to smooth
out the surface and reduce the dose inhomogeneity.
116. The bolus also used to reduce the electron beam
penetration In some parts of the treatment field
117. FIELD MATCHING
• Occasionally, situations arise in which adjacent fields must
be matched in order to avoid an over- or under-dose.
Examples of situations in which such beam matching may be required
• 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
118. When two adjacent electron fields are overlapping or
abutting, there is a danger of delivering excessively high
doses in the junction region.
On the other hand, separating the fields may seriously
under dose parts of the tumor
119. Electron fields abutting
• Because the tumors treated with electrons are mostly
superficial, the electron fields are usually abutted on
• If the edges of the two
beams coincide exactly,
then the two beams will
be equivalent to a single
beam and optimal uniformity
will be achieved
Common edge is created by having diverging
120. • If the central axes of the two beams are parallel , then
the diverging beam edges will overlap, creating a cold
spot upstream and a hot spot downstream of the
region of intersection.
Parallel central axes result in overlapping
121. • If the central axes are converging , then there is an
even greater amount of overlap. This is the case for
abutting lateral and medial chest-wall fields.
C: Converging central axes result in the
122. • In a clinical situation, the decision as to whether the
fields should be abutted or separated is based on the
uniformity of the combined dose distribution across the
• The large penumbra and bulging isodose lines make
hot spots and cold spots in the target volume practically
• the resulting hot spots are taken into account in
evaluating the treatment plan.
124. MIXED BEAM THERAPY
• Electron beams frequently are mixed with photon
beams to create an appropriate treatment.
• When an electron field is abutted at the surface with a
photon field, a hot spot develops on the side of the
photon field and a cold spot develops on the side of
the electron field .
• This is caused by out scattering of electrons from the
125. • A 9-MeV electron field is abutted with a 6-MV photon
field - treatment of tumors in the neck.
• Whereas the photon field is used to treat the anterior
neck, the electron field is used to treat the posterior
neck nodes overlying the cord.
• Because of the limited range of the electrons, the cord
can be spared, while sufficient dose can be delivered to
126. • the electron beam is incident at the standard SSD of
100 cm, with the distance between the applicator end
and the surface being 5 cm.
127. • The electron beam is
incident at an extended
SSD of ~ 120 cm
• Reveals that the extent of
hot and cold spots depends
on the electron beam SSD
• The increased air gap between the applicator and the
surface causes the electron beam profile to become
less flat as a result of increased scattering of electrons
128. • Consequently, the hot and cold spots spread out to
cover larger areas, without significantly changing
129. Electron ARC Therapy
• Electron arc therapy is a special radio-therapeutic
technique in which a rotational electron beam is used
to treat superficial tumor volumes that follow curved
• On the basis of isodose distribution, electron arc
therapy is most suited for treating superficial volumes
that follow curved surfaces such as :
The chest wall, ribs, and entire limbs
130. • it is not widely used because it is relatively
complicated and its physical characteristics are poorly
• The calculation of dose distributions in electron arc
therapy is a complicated procedure and usually
cannot be performed reliably with the algorithms
used for standard stationary electron beam treatment
131. • Not all electron accelerators are equipped with electron
• Linear accelerators are being made with this capability.
Besides the arcing capability, certain modifications in
electron collimation are necessary to make this