This document discusses key concepts in external beam therapy including: 1) Photon fluence, photon fluence rate, kerma, exposure, and absorbed dose which are quantities used to describe photon beams. 2) Important radiation treatment parameters including depth of treatment, field size, source-skin distance, source-axis distance, beam energy, number of beams, and monitor units. 3) Percentage depth dose curves which provide information about beam quality and energy based on how the absorbed dose changes with depth for different field sizes and energies.

1. External Beam Therapy (EBT)

2. Lecture 5

3. 1- Quantities used in describing a
photon beam

4. Photon fluence and photon fluence rate
Photon fluence
• dN is the number of photons that enter an
imaginary sphere of cross-sectional area dA.
• Unit of photon fluence is cm-2.

5. • Photon fluence and photon fluence rate
- Photon fluence rate is defined as the photon
fluence per unit time.
• Unit of photon fluence rate is cm-2. s-1.
Photon fluence and photon fluence rate

6. Kerma
Kerma (Kinetic Energy Released per unit Mass)
The quantity kerma is the kinetic energy released
in the medium
Kerma is defined as:
K =
dEtr is the sum of the initial kinetic energies of all
the charged particles liberated by uncharged
particles in a mass dm
dEtr
dm
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7. Exposure
Exposure is:
A quantity used to indicate the amount of
ionization in air produced by x- or gamma-ray
radiation
The unit of exposure is the coulomb per kilogram
(C/kg)
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8. Absorbed Dose
The absorbed dose, D, is given by:
D = de/dm
Where de is the mean energy imparted to
matter of mass dm
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9. Absorbed Dose
 The unit of absorbed dose is J kg-1
 The special name for the unit of absorbed
dose is gray (Gy)
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10. 2- Radiation treatment parameters

11. The main parameters in external beam dose delivery with
photon beams are:
• Depth of treatment z.
• Fields size A.
• Source-skin distance (SSD) in SSD setups.
• Source-axis distance (SAD) in SAD setups.
• Photon beam energy
• Number of beams used in dose delivery to the patient.
• Treatment time for orthovoltage and teletherapy machines.
• Number of monitor units (MUs) for linacs.

12. Depth of dose maximum zmax
The depth of dose maximum zmax under the patient’s surface
depends on the beam energy and beam field size.
The beam energy dependence is the main effect; the field size
dependence is often ignored because it represents only a minor
effect.
Nominal values for zmax range from zero for superficial and
orthovoltage X ray beams, through 0.5 cm for 60Co beams, to 5 cm
for 25 MV beams.

13. Radiation beam field size
Four groups of field shape are used in radiotherapy:
• Square (produced with collimators installed in therapy
machine).
• Rectangular (produced with collimators installed in
therapy machine).
• Circular (produced with special collimators attached to
treatment machine).
• Irregular (produced with custom made shielding blocks or
with multileaf collimators).
Field sizes range from small circular fields used in radiosurgery,
through standard rectangular and irregular fields, to very large
fields used for total body irradiation (TBI).

14. SSD and SAD are geometric parameters that can be
changed, and the ability to change them leads to two
different planning and treatment set-up modalities
:a constant source to surface distance (SSD) for all
beams or an isocentric set-up with a constant source to
axis distance (SAD).
 In an SSD set-up, the distance from the source to the
surface of the patient is kept constant for all beams,
while for an SAD set-up the centre of the target
volume is placed at the machine isocentre;
(SSD) &(SAD).

15. (SSD) &(SAD).

16. Percentage Depth Dose (PDD)
• Is The absorbed dose at any
depth Q to the absorbed dose
at a fixed reference depth P,
along the central axis of the
beam . See figure
PDD =
DQ
DP
X 100

17. Percentage Depth Dose (PDD)
The percentage depth dose curve provides
information on the quality of the radiation and its
energy.
The percentage depth dose depends on four
parameters:
• Depth in phantom z
• Field size A on patient’s surface
• Source-surface distance SSD
• Photon beam energy

18. Monitor Unit (MU) Calculation in a Linear
Accelerator
Monitor units are the units in which the output of a linac is
measured.
Linacs are calibrated to give 1 cGy at a SAD distance of 100
cm, for a field size of 10 × 10 cm, and at the depth
corresponding to Dmax, and this calibration dose is defined as
one monitor unit (MU).
Percent depth dose is a suitable quantity for calculations
involving SSD techniques.
Machines are usually calibrated to deliver 1 rad (10-2 Gy) per
monitor unit (MU) at the reference depth t0, for a reference
field size 10 × 10 cm and a source to calibration point distance
of SCD.

19. Thank you for your attention