1) Electron beam therapy uses high-energy electrons between 6-20MeV to treat superficial tumors less than 5cm deep. It is useful for cancers of the skin, eye, breast, head and neck, and gastrointestinal tract. 2) Electron beams have distinct advantages over x-rays and brachytherapy in minimizing dose to deeper tissues and providing dose uniformity. 3) The depth that receives 90% of the maximum dose, called R90, is typically one-third to one-fourth of the electron energy in MeV. This determines the maximum treatment depth.

1. ELECTRON BEAM THERAPY
DR ABANI KANTA NANDA
1ST YEAR PG STUDENT
DEPT. OF RADIOTHERAPY
AHRCC, CUTTACK

2. DEFINITION
• Electron Beam Therapy is a modality of radiation therapy where the high
energy electrons of energy range 6-20MeV used for treating superficial
tumor <5cm deep.

3. Skin: Lip Ear
Eyelids Scalp
Nose limbs
Upper-respiratory and digestive tract: floor of mouth
soft palate
retromolar trigone
salivary glands
Breast: chest-wall irradiation following mastectomy
nodal irradiation
boost to the surgical bed
Other sites: Retina
Orbit
spine (craniospinal irradiation)
pancreas and other abdominal structures
(intraoperative therapy)
cervix (intracavitary irradiation)
Electrons are useful in treating cancer of

4. WHY UNIQUE ?
• Distinct advantages over
• Superficial XRay
• Brachytherapy
• Tangential photon beam
Minimising dose to deeper tissue
Dose uniformity
• Hence G.H. Fletcher said-
There is no alternative to electron
beam therapy.

5. ELECTRON INTERACTION
• Inelastic collision with electron
– Ionisation and excitation
• Inelastic collision with atomic nuclei
– Bremsstrahlung
• Elastic collision with electron
– Electron electron scattering
• Elastic collision with atomic nuclei
– Nuclear scattering

6. • An electron travelling in a medium loses energy as a result of
collisional and radiative processes
• Collisional loses(ionisation and excitation)
The rate of energy loss depends on the electron
density of the medium
The rate of energy loss per gram per centimetre
squared, which is called mass stopping power , is greater for
low atomic number(z) material
• Radiation loses (bremsstrahlung)
The rate of energy loss per centimetre is
approximately propertional to the electron energy and to the
square of atomic number(z²).

7. ELECTRON SCATTERING
• When a beam of electrons passes through a medium, the electrons suffer
multiple scattering due to interactions between the incident electrons and
the nuclei of the medium.
• So the electrons acquire velocity components and displacements
transverse to their original direction of motion.
• The scattering power varies
 Directly as the square of the atomic number
 inversely as the square of the kinetic energy
• For this reason, high atomic number materials are used in the construction
of scattering foils.

8. DOSE DISTRIBUTION IN WATER
ELECTRON VS PHOTON

9. DEPTH DOSE
• Depth in centimetre at which electron deliver
a dose to the 80 -90% isodose level is equal
to approximately one third to one fourth of
electron energy in MeV.
• Most useful treatment depth or theraputic
range of electron is given by depth of 90%
depth dose
• The principle is that when in doubt , use a
high energy electron to make sure that the
target volume is well with in the specified
isodose curve

10. •Rp IS THE PRACTICAL RANG
•R50 IS THE DEPTH AT WHICH THE
DOSE IS 50% OF THE MAXIMUM
DOSE

11. •MEAN ENERGY- IT HAS BEEN SHOWN THAT THE MEAN ENERGY
OF THE ELECTRON BEAM, E0, AT THE PHANTOM SURFACE IS
RELATED TO R50 BY THE FOLLOWING RELATIONSHIP:
E0=C4 . R50
(WHERE C4 = 2.33 MeV/CM FOR WATER)
•ENERGY AT DEPTH-THE MEAN ENERGY OF THE SPECTRUM
DECREASE LINEARLY WITH DEPTH. THIS CAN BE EXPRESSED BY
THE RELATIONSHIPS:
WHERE Z IS THE DEPTH

12. ENERGY DEPENDENCY ON DEPTH
DOSE
• Percent depth dose increases as
energy increases.
• However percent surface dose for
electrons increases with energy.

13. PHANTOMS
• Water is the standard phantom for the dosimetry of electron beams.
• However, it is not always possible or practical to perform dosimetry in a
water phantom.
• It also is difficult to make measurements near the surface of water,
because of its surface tension and the uncertainty in positioning the
detector near the surface.
• For a phantom to be water equivalent for electron dosimetry it must ‘has
the same electron density (number of electrons per cubic centimeter) and
the same effective atomic number as water.

14. • An effective density may be assigned to a medium to give water-equivalent
depth dose distribution near the therapeutic range
• It has recommended that the water equivalent depth or the effective
density (ρeff) may be estimated from the following relationship:
• Where R50 is the depth of 50% dose

15. • Recommended values of effective density for
various phantoms are given in the table:

16. DOSE DISTRIBUTION IN PATIENT
• The ideal irradiation condition is for electron beam to be incident normal
to a flat surface with underlying homogeneous normal tissue
• When the angle of incidence deviates from normal ,surface becomes
irregular, internal heterogenous tissue present, the qualities of dose
distribution deviates from that in phantom.
• Internal heterogenity can change depth of beam penetration
• Both irregular surface and internal heterogenities create changes in side
scatter equilibrium, producing volume of increased dose(hot spot) and
decreased dose(cold spot)

17. OBLIQUE INCIDENCE
• For obliquely incidence beams whose
angle of incidence is greater than 30°,
there is a significant change in shape of
central axis percent depth dose.
• Compared with normal incidence, the
oblique incident electron beam shows
the following
(a) an increased surface dose,
(b) an increased maximum dose,
(c) a decreased penetration of the
therapeutic dose (R90),
(d) an increased range of penetration
• Clinical examples- treatment of chest
wall, limbs, scalp

18. SURFACE IRREGULARITIES
• Sharp surface irregularities produced
localised hot spots and cold spots in
the underlying medium due to
scattering
• Irregular skin surfaces are
encountered primarily during
treatment of nose, eye, ear, ear canal
and in groin area
• Surgical excision creates treatment
area with abrupt changes in surface in
body

19. TISSUE HETEROGENEITY
• Dose distribution can be significantly alter in presence of tissue
inhomogeneities such as bone, lung and air cavities.
• Electron depth dose distribution in a medium are depends on electron
densities( electron/cm³)
• For beam passing through lung material of densities 0.25g/cm³ depth of
penetration in lung would be
Z(lung)=z(water)*ρ(lung)
(Z= Depth of penetration, ρ=density)
• Thus a beam that would penetrate 1cm normal unit densities material
such as water, would penetrate 4cm depth in lung.

20. • Left figure shows beam incidence on
chest wall with out taking the density
of lung into account
• Right figure shows the dramatic
increase in dose to lung when this
inhomogeneity is taken into account
(For simplicity the effect of ribs
have not been considered )

21. TREATMENT PLANNING IN ELECTRON
BEAM
• The first step in the initiation of electron therapy is to determine
accurately the target to be treated.
• The electron energy for treatment should be selected such that the depth
of 90% isodose line covers the deepest portion of the region to be treated
in addition to an approximate 5mm additional depth beyond treatment
region

22. • Isodose curves are lines passing through points of equal dose.
• Isodose curves are usually drawn at regular intervals of absorbed dose and
are expressed as a percentage of the dose at a reference point, which is
normally taken as the Zmax point on the beam central axis.
• 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, field size and beam collimation.

23. ELECTRON APPLICATORS
Electron beam applicators or cones are usually used to collimate
the beam, and are attached to the treatment unit head.

24. • Normally the photon beam collimators on the accelerator are too far from
the patient 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, and in the air
between the exit window and the patient, to create a clinically
unacceptable penumbra. Hence electron applicators are used.
• Several cones are provided, usually in square field sizes ranging from 5 × 5
cm² to 25 × 25 cm².

26. • 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.
• Standard cut-out shapes may be preconstructed and ready for use at the
time of treatment.
• Custom cut-out shapes may also be designed for patient treatment. Field
shapes may be determined from conventional or virtual simulation, but
are most often prescribed clinically by the physician prior to the first
treatment.

27. 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 is the most common material used for production of internal shield
because of its availability and ease of use

28. • The required thickness of the shield depends on energy of electron beam,
the fact that
 electron decrease in energy by 2MeV/cm in muscle
 1mm of lead is required as shielding for every 2MeV of energy
(plus 1mm for safety)
• Thus 9MeV of electrons are used to treat the buccal mucosa of thickness
1cm, a shield placed beneath the cheek to protect oral cavity would have
to be 4.5mm thick.
• This is because the electrons would decrease to 7MeV after penetrating
1cm of tissue, and 3.5+1=4.5mm of lead would be required to shield
7MeV electrons

29. • Aluminium or acrylic materials have been used around lead shields to
absorb the backscattered electrons.
• Often, these shields are dipped in wax to form a 1 or 2 mm coating around
the lead. This not only protects the patient from the toxic effects of the
lead, but also absorbs any scattered electrons, which are usually low in
energy

30. BOLUS
• Bolus is often used in electron beam therapy for the following purposes.
o To increase the surface dose;
o To flatten out irregular surfaces;
o To reduce the electron beam penetration in
Some parts of the treatment field.
• Several tissue equivalent materials are used for bolus such as –
paraffin wax
polystyrene
acrylic(pmma)
super stuff
superflap
superflex

31. Construction of a custom bolus to comform isodose lines to
the shape of the target

32. • 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.
• The use of bolus for electron beam treatments is very practical, since
treatment planning software for electron beams is limited and empirical
data are normally collected only for standard beam geometries.

33. FIELD ABUTMENT
• The purpose of field abutment usually is to enlarge the radiation field or to
change the beam energy or modality

34. • This figure shows a comparison of
several abutting beam configuration
• Fig-a—The extent and magnitude of
the high dose region can be minimised
by angling the central axis of each
away from each other so that a
common beam angle is formed
• Fig b—Represents overlap that can
occur when the central axis of beams
are parallel
• Fig c—Shows converging beam central
axes that result in the greatest amount
of overlap with the highest doses and
largest high dose regions

35. ELECTRON ARC THERAPY
• Electron arc therapy is a special radio therapeutic technique in which a
rotational electron beam is used to treat superficial tumour volumes that
follow curved surfaces.
• While the technique is well known and accepted as clinically useful in the
treatment of certain tumours, it is not widely used because it is relatively
complicated and its physical characteristics are poorly understood.

36. • 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
planning.
• It is useful in treating postmastectomy chest wall, usually in barrel chested
women where tangent beam can irradiate too much lung.

37. • There are three levels of collimation
in electron arc therapy:
 the primary x-ray collimators
 a shaped secondary cerrobend insert
 skin collimation
• The treatment-planning process for
arc therapy requires
 patient CT scanning
 delineation of PTV
 selection of isocenter location
 specification of electron arc
boundaries
 energy and slab bolus selection
 design of secondary collimator
 calculation of dose

38. INTRACAVITARY IRRADIATION
• It is performed for treatment of
 intraoral (such as floor of mouth, tongue, soft palate, retromolar trigone)
 transvaginal areas
• Intraoperative radiotherapy is also an intracavitary irradiation
• For intracavitary irradiation specially design cones and adaptors are
required
cones cone with adaptor

39. INTRAOPERATIVE ELECTRON BEAM
RADIATION THERAPY(IOERT)
• IOERT is a treatment option that delivers a concentrated, precise dose of
radiation during surgery, immediately after a tumor is removed.
• Intraoperative treatments are performed completely in the operating
room.
• Benefits
 One to two minutes IOERT can reduce the likelyhood of tumor recurrence
and increase cure rates.
 It also lessen the need for post operative external radiotherapy

40. TOTAL-LIMB IRRADIATION
• It may be advantageous to irradiate the superficial anatomy of a limb for
management of cancer , e.g., melanoma
lymphoma
Kaposi’s sarcoma
• If the depth beneath the surface is 2 cm or less, electrons offer a uniform
dose while sparing deep tissues and structures.
six equally spaced 5-MeV electron
beams are used to irradiate a 9cm
diameter cylinder.

41. TOTAL SKIN IRRADIATION
• Total-skin electron irradiation is a modality designed for
management of diseases that require irradiation of the entire skin
surface or a significant portion of it.
• The technique is used most frequently for treatment of
 mycosis fungoides
 kaposi’s sarcoma.
• Multiple techniques for total-skin electron therapy have been
reviewed.
• The underlying principles of the various techniques are similar,
which are exemplified by the modified stanford technique.

42. Schematic of modified Stanford technique.
Side view of setup shows the relative position
of patient plane, scatter plate, isocenter,
and gantry angles (A).Six beam directions (B)
are achieved by placing the patient in six
patient positions (C)

43. TOTAL SCALP IRRADIATION
• Total-scalp irradiation is sometimes necessary in the management of
malignancies (e.g., cutaneous lymphoma, melanoma, and angiosarcoma)
that present with widespread involvement of the scalp and forehead.
• Electron-beam therapy is a practical means of achieving the therapeutic
goal of delivering
 a uniform dose to the scalp
 with minimal dose to underlying brain.

44. SUMMARY
• Electron beam therapy is used for treating superficial tumor.
• It’s benefits are minimum dose to deeper tissue and dose uniformity.
• Water or other water equivalent phantoms are used to study properties of
electron beam therapy.
• Oblique incidence, irregular surface and tissue heterogeneity must be
consider while planning for electron beam therapy.
• Electron applicators, standard cut outs or custom cut outs are used in
Linac for electron beam therapy

45. • Always use a high energy electron to make sure that the target volume is
well within the specified isodose curve
• Electron arc therapy, intracavitary irradiation, total limb irradiation, total
skin irradiation and total scalp irradiation are modalities of treatments
where electron beams are used to treat superficial tumors.