1) Megavoltage photon therapy uses high energy photon beams, typically above 1 MV, generated via linear accelerators or cobalt-60 sources. 2) The three main interactions between photons and tissue are the photoelectric effect, Compton scattering, and pair production. Compton scattering is the most common interaction for megavoltage beams between 6-20 MV. 3) Key characteristics of megavoltage photon beams include percentage depth dose curves, beam profiles, isodose charts, and considerations for tissue heterogeneities. Wedge filters can also be used to modify beam profiles for uniform target coverage.

1. Clinical and Physical characteristics
of Megavoltage Photon Therapy
Presenter : Dr. Gowtham Manimaran
Moderator : Mr. Malhotra

2. IONIZING RADIATION :
Ionizing radiation is energy sufficiently strong to remove an orbital
electron from an atom.
This radiation can have an electromagnetic form, such as a high-energy
photon, or a particulate form, such as an electron, proton, neutron, or
alpha particle.

3. 1. Photons
By far, the most common form of radiation used in practice today is the high-
energy photon
• packets of energy.
• With the development of relativity, scientists discovered that waves also
have a mass. It is because waves behave as particles on interactions with
matter.
• However, the rest mass of a photon is zero
Origin
Tissue-photon interactions
Physical and clinical Characteristics

4. Origin
• Gamma rays :
originate from radioactive nuclei
usually isotropic and produce monoenergetic photon beams
Ex : Co teletherapy , brachytherapy
• Which originate from targets (bombarded by electrons)
These consist of bremsstrahlung photons and characteristic photons
Non-isotropic sources producing heterogeneous photon spectra

5. Bremsstrahlung Photons
• Bremsstrahlung (braking radiation) is a result of
radioactive collision between a high speed
electron and a nucleus
• Direction of emission of bremsstrahlung photons
depends on the energy of the incident electrons
Characteristic Photons
• An electron, with kinetic energy E0, may interact
with the atoms of the target by ejecting an orbital
electron, such as a K, L, or M electron, leaving the
atom ionized. The original electron will recede
from the collision with energy E0 – ΔE, where ΔE is
the energy given to the orbital electron. A part of
ΔE is spent in overcoming the binding energy of
the electron and the rest is carried by the ejected
electron. When a vacancy is created in an orbit, an
outer orbital electron will fall down to fill that
vacancy. In so doing, the energy is radiated in the
form of electromagnetic radiation.

6. PHOTON-TISSUE INTERACTIONS
• The Photoelectric effect
• Compton effect and
• Pair production.

7. The Photoelectric effect
• In this process, an incoming photon undergoes a collision with a
tightly bound electron.
• The electron departs with most of the energy from the photon and
begins to ionize surrounding molecules.
• This interaction depends on the energy of the incoming photon, as
well as the atomic number of the tissue
• Ex :Diagnostic x-ray film. Since the atomic number of bone is 60%
higher than that of soft tissue, bone is seen with much more
contrast and detail than is soft tissue. The energy range in which the
photoelectric effect predominates in tissue is about 10-25 keV.

8. The Compton effect
• The Compton effect is the most important photon-tissue interaction for the
treatment of cancer. In this case, a photon collides with a “free electron,” ie, one
that is not tightly bound to the atom.
• Unlike the photoelectric effect, in the Compton interaction both the photon and
electron are scattered. The photon can then continue to undergo additional
interactions, albeit with a lower energy. The electron begins to ionize with the
energy given to it by the photon.
• The probability of a Compton interaction is inversely proportional to the energy
of the incoming photon and is independent of the atomic number of the material.
• The Compton effect is the most common interaction occurring clinically, as most
radiation treatments are performed at energy levels of about 6-20 MeV.
• Ex: Port films are films taken with such high-energy photons on the treatment
machine and are used to check the precision and accuracy of the beam; because
they do not distinguish tissue densities well, however, they are not equal to
diagnostic films in terms of resolution.

9. The Pair production
• In this process, a photon interacts with the nucleus of an atom.
• The photon gives up its energy to the nucleus and, in the
process, creates a pair of positively and negatively charged
electrons. The positive electron (positron) ionizes until it
combines with a free electron. This generates two photons that
scatter in opposite directions.
• The probability of pair production is proportional to the
logarithm of the energy of the incoming photon and is
dependent on the atomic number of the material. The energy
range in which pair production dominates is ≥ 25 MeV.

10. MEGAVOLTAGE THERAPY
X-ray beams of energy 1 MV or greater can be classified as megavoltage
beams.
Although the term strictly applies to the x-ray beams, the γ-ray
beams produced by radionuclides are also commonly included in this
category if their energy is 1 MeV or greater.
Examples :Van de Graaff generator, linear accelerator, betatron and
microtron, and teletherapy γ-ray units such as cobalt-60.

11. Characteristics
• A photon beam propagating through air or a vacuum is governed by
the inverse square law; a photon beam propagating through a
phantom or patient, on the other hand, is affected not only by the
inverse square law but also by the attenuation and scattering of the
photon beam inside the phantom or patient. These three effects
make the dose deposition in a phantom or patient a complicated
process and its determination a complex task.
(Inverse square law : This law states that the radiation intensity from a
point source is inversely proportional to the square of the distance
away from the radiation source. In other words, the dose at 2 cm will
be one-fourth of the dose at 1 cm)

12. • Basic dose distribution data are usually measured in a water
phantom, which closely approximates the radiation absorption and
scattering properties of muscle and other soft tissues. Another reason
for the choice of water as a phantom material is that it is universally
available with reproducible radiation properties.
PDD
Profiles
Tissue interface dosimetry
Wedge , compensating filter and bolus

13. • A typical dose distribution on the
central axis of a megavoltage photon
beam striking a patient is shown.
• The beam enters the patient on the
surface, where it delivers a certain
surface dose Ds. Beneath the surface
the dose first rises rapidly, reaches a
maximum value at depth zmax
• Then decreases almost exponentially
until it reaches a value Dex at the
patient’s exit point.

14. PDD
• 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

15. • The central-axis PDD expresses the
penetrability of a radiation beam.
• Factors affecting PDD:Field size ,
energy of the beam
• As arule of thumb, an 18-MV , 6-MV
, and 60Co photon beam loses
approximately 2%, 3.5%, And 4.5%
per cm, respectively, beyond the
depth of maximum dose,
dmax(values are for a 10- × 10-cm
field, 100-cm SSD)

16. Skin sparring effect
• For megavoltage photon beams the surface dose is generally much lower than
the maximum dose, which occurs at a depth zmax beneath the patient’s surface.
• In megavoltage photon beams the surface dose depends on the beam energy
and field size.
• The larger the photon beam energy, the lower the surface dose, which for a 10 ×
10 cm2 field typically amounts to some 30% of the maximum dose for a cobalt
beam, 15% for a 6 MV X ray beam and 10% for an 18 MV X ray beam.
• For a given beam energy the surface dose increases with the field size.
The low surface dose compared with the maximum dose is referred to as the skin
sparing effect and represents an important advantage of megavoltage beams over
orthovoltage and superficial beams in the treatment of deep seated tumours.

17. Beam Profile
The variation of dose occurring on a line perpendicular to
the central beam axis at a certain depth is known as the
beam profile. It represents how dose is altered at points
away from the central beam axis. There are typically three
parts:
• The central region -which is usually flat and includes
doses over 80% of the central beam axis.
• The penumbra region where dose falls off rapidly at the
beam edge, between a dose of 20-80% of the central
beam axis.
• The umbra region where dose is minimal (under 20% of
the central beam dose)
• Other distinctive features are the lateral horns, which
are only present in photon beams – caused by the
flattening filters (and more pronounced for 18 MV
photons).

19. Isodose curves
• 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).

20. Isodose chart Co60 10*10cm
SSD =80cm
SAD

21. Isodose charts

22. • Beam characteristics for x-
ray and γ-ray beams typically
usedin radiation therapy and
lists the depth at which the
doseis maximum (100%) and
the 10-cm depth PDD value.

23. Tissue heterogeneities and Tissue interface
dosimetry
• The presence of tissue heterogeneities, such as air cavities, lungs,
bony structures, and prostheses, can greatly impact the calculated
dose distribution. The change in dose is due to the perturbation of
the transport of primary and scattered photons and that of the
secondary electrons set in motion from photon interactions.
• Depending on the energy of the photon beam and the shape, size,
and constituents of the inhomogeneities, the resultant change in
dose can be large.

24. Wedge Dosimetry
• When a wedge filter is inserted into the
beam, the dose distribution is angled at
some specified depth to some desired
angle
• To assist in obtaining uniform irradiation
throughout the treated volume,
techniques have been developed using
wedge filters, i.e., non-uniform (metal)
filters whose thickness varies in one
direction across the field.
• Linacs are typically equipped with
multiple wedges that may be used , (MC
15 ,30 ,45 and 60 degree wedges)
• Enhanced dynamic wedges

25. Compensating
filter
• Counteracts the effects caused by variations in
patient surface curvature while still preserving
the desirable skin-sparing feature of
megavoltage photon beams.
• This is accomplished by placing the custom-
designed compensating filter in the beam,
sufficiently “upstream” from the patient’s
surface.

26. Bolus
• Tissue equivalent material placed directly on the patient’s skin surface
to reduce the skin sparring effect of MV Photons
• Should ideally have : similar electron , physical , atomic number
properties to those of water/tissue , should be pliable , Inexpensive
• Commonly used ones are,
 Paraffin wax
 Rice bags filled with Soda
 Gauze coated with petrolatum
 Synthetic substances-Superflab ,Superstuff
• Adding bolus may also fill tissue defect to smooth irregular
distribution , can also alter dose distribution

27. Thank You