This document discusses central axis depth doses in water for both SSD and SAD techniques. For SSD technique: - Percentage depth dose (PDD) curves measure attenuation at different depths and are affected by beam quality, field size, and SSD. - Buildup region occurs as secondary electrons deposit energy downstream, increasing dose with depth until maximum. - Depth dose maximum (zmax) depends on beam energy and field size. - PDD increases with larger field sizes due to increased scatter radiation. - PDD increases with longer SSD due to the inverse square law of radiation intensity.

1. 3.2 Central Axis Depth Doses In Water
3.2.1 Central Axis Depth Doses In Water SSD Technique
3.2.1.1 Percentage Depth Dose (PDD)
3.2.1.1.1 Effect of Beam Quality & Depth
3.2.1.1.2 Effect of Field Size & Shape
3.2.1.1.3 Effect of SSD
3.2.2 Central Axis Depth Doses In Water SAD Technique
3.2.2.1 Tissue Air Ratio (TAR)
3.2.2.1.1 Effect of Distance
3.2.2.1.2 Effect of energy, depth, and field size
3.2.2.1.3 TAR and SAD
3.2.2.1.4 BSF
3.2.2.2 Tissue Phantom Ratio (TPR)
3.2.2.3 Tissue Maximum Ratio (TMR)
3.2.2.4 Scatter Air Ratio (SAR)
PART 2
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2. 3.2.1 Central Axis Depth Doses In Water
SSD Technique
228/1/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

3. SOURCE TO SKIN DISTANCE
Part VIII.3.5 Determination of Dose to a Patient-I Slide 3
SSD=80cm
What is SSD?

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What is SSD?
Kilovoltage machines are
typically fixed and therefore
treatment is based using a
constant source – surface
distance (SSD). SSD
calculations use percentage
depth dose curves and are
easier to measure in a
phantom.
With more modern
treatments using multiple
fields, the use of a constant
SSD technique leads to
frequent patient
repositioning between
treatments.

5. Part VIII.3.5 Determination of Dose to a Patient-I Slide 5
•SSD is the acronym of Source to Skin distance. It is
the distance between the source and the patient skin
•Machines have Standard SSD at which output and
PDD are measured
•50 cm for a Cs 137 Unit
•80 – 100 cm for Co 60 unit
•100 cm for Linear accelerator
What is SSD?

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7. Why is SSD important?
• The dose calibration of the External beam unit is at the SSD
(or Isocentre – explained later)
• Any change in this will vary the dose by ‘inverse square’
factor
Part VIII.3.5 Determination of Dose to a Patient-I Slide 7
What is SSD?

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3.2.1.1 Percentage Depth Dose (PDD)
3.2.1.1.1 Effect on Beam Quality & Depth
3.2.1.1.2 Effect on Field Size & Shape
3.2.1.1.3 Effect on SSD

9. 1. What it is?
• Attenuation factors
• Simple to measure and used for dose
calculation
• Measured at SSD
3.2.1.1 Percentage Depth Dose (PDD)

10. •
•
2. PDD Measurement
Photon beam hits a rectangular phantom
Photon beam get attenuated by the phantom
material.
The beam intensity falls as the beam is
attenuated by the phantom
•
3.2.1.1 Percentage Depth Dose (PDD)

11. • Absorbed dose at any depth: Dd
• Absorbed dose at a fixed reference depth: Dd0
100
0

d
d
D
D
P
collimator
surface
phantom
D d0
D d
d
d0
3.1.3 Percentage Depth Dose (PDD)
Quotient expressed as percentage, of the absorbed dose at any depth d to the absorbed
dose at dmax along the central axis of the beam
3.2.1.1 Percentage Depth Dose (PDD)

12. Dose distribution
3.1.3 Percentage Depth Dose (PDD)3.2.1.1 Percentage Depth Dose (PDD)

13. For higher energies, the
reference depth is at the
peak absorbed dose
( d 0= d m)
D max : maximum dose of
the given dose
For orthovoltage (up to 400 kVp)
and lower energy X-rays, the
reference depth is usually the
surface (d 0= 0(position))
3.1.3 Percentage Depth Dose (PDD)3.2.1.1 Percentage Depth Dose (PDD)

14. Part VIII.3.7 Operational Considerations – Planning of physical
treatment
Slide 14
3.1.3 Percentage Depth Dose (PDD)3.2.1.1 Percentage Depth Dose (PDD)
3. Dose deposition from a megavoltage
photon beam in a patient

15. Beam enter the patient
Deliver surface dose, Ds
Dose rise rapidly after the surface
dose , build up region
Reach maximum value at depth zmax
Decrease exponentially
Reaches value Dex at the patient exit
point
3.2.1.1 Percentage Depth Dose (PDD)
3. Dose deposition from a megavoltage
photon beam in a patient

16. a.Surfacedose
• For MV photon beam, surface dose is lower than
maximum dose
• Depends on:
– Beam energy ( beam energy, surface dose)
• E.g: 15% for 6 MV, 10 % for 18 MV
– Field size ( field size, surface dose)
3.2.1.1 Percentage Depth Dose (PDD)

17. • Low surface dose is also called skin sparing effects
• Important advantage of MV beams over orthovoltage
and superficial beams in the treatment of deep seated
tumours
• KV beam do not exhibit skin sparing effects since their
maximum dose occur on the skin surface (surface dose
maximum dose)
3.2.1.1 Percentage Depth Dose (PDD)
a.Surfacedose

18. • The surface dose represent contributions to the
dose from:
–Photons scattered from the collimators, flattening
filter and air
–Photons backscattered from the patient
–High energy electrons produced by photon
interaction in air and any shielding structures in
vicinity of the patient.
3.2.1.1 Percentage Depth Dose (PDD)
a.Surfacedose

19. Superficial beam
Orthovoltage
beam
3.2.1.1 Percentage Depth Dose (PDD)
a.Surfacedose

20. B. Buildupregion
• The dose region between the surface (depth z=0) and depth z= zmax)
• Resulting from long range of energetic secondary charged particles that
first released in the patient by photon interaction (photoelectric effects,
compton effects, pair production)
• Then the kinetic energy deposited in the patient. Therefore, the
electron fluence and hence the absorbed dose increase with depth
until they reach a maximum.
3.2.1.1 Percentage Depth Dose (PDD)

21. Result of the forward direction
of secondary electrons - they
deposit energy down stream
from the original interaction
point
B. Buildupregion
3.2.1.1 Percentage Depth Dose (PDD)

22. B. Buildupregion
3.2.1.1 Percentage Depth Dose (PDD)

23. 3.1.3 Percentage Depth Dose (PDD)
DOSE BUILD-UP
3.2.1.1 Percentage Depth Dose (PDD)

24. • In the region immediately beneath patient’s surface, the
condition of CPE does not exist
• Absorbed dose < collision kerma
• As depth increase, CPE will be achieved at zmax, where z is
equal to the range of secondary charged particles
• At this stage, dose become equal to collision kerma.
• Beyond zmax, the dose and collision kerma decrease because of
of photon attenuation in the patient (transient CPE)
B. Buildupregion
3.2.1.1 Percentage Depth Dose (PDD)

25. (1) Kinetic Energy Released In The Medium;
(2) the energy transferred from photons to directly ionizing electron;
(3) maximum at the surface and decreases with depth due to the
decrease in the photon energy fluence;
(4) the production of electrons also decreases with depth
KERMA
3.1.3 Percentage Depth Dose (PDD)3.2.1.1 Percentage Depth Dose (PDD)
B. Buildupregion

26. ABSORBED DOSE
(1) depends on the electron fluence
(2) high-speed electrons are ejected from the
surface and subsequent layers
(3) these electrons deposit their energy a
significant distance away from their site of origin
3.1.3 Percentage Depth Dose (PDD)3.2.1.1 Percentage Depth Dose (PDD)
B. Buildupregion

27. C. Depth of dose maximum zmax
• Beneath patient surface
• Depends :
– Beam energy
– Beam field size (minor effects)
3.2.1.1 Percentage Depth Dose (PDD)

28. D. Exit dose
• The dose delivered to the patient at the beam exit point
• Dose distribution curve slightly downwards from the
extrapolated dose distribution curve
• Because of small effect due to the missing scatter
contribution at the exit point from beyond the exit dose
point
3.2.1.1 Percentage Depth Dose (PDD)

29. Percentage Depth Dose (electron)
PDD of electron beam

30. Isodose curves of
electron beam
• Scattering of electrons determines
shapes of isodose curves
–Expansion
–Lateral constriction
• Larger field size required at surface
3.1.3 Percentage Depth Dose (PDD)

31. 3.1.3 Percentage Depth Dose (PDD)

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3.2.1.1.1 Effect on Beam Quality & Depth
3.2.1.1.2 Effect on Field Size & Shape
3.2.1.1.3 Effect on SSD

33. Dependence of PDD
PDD
(A) DEPENDENCE ON BEAM QUALITY AND
(B) EFFECT OF FIELD SIZE AND SHAPE
(C) DEPENDENCE ON SSD

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3.2.1.1.1 Effect on Beam Quality & Depth

35. 3.2.1.1.1 Effect on Beam Quality & Depth
PDD (beyond depth of
max. dose) increases
with beam energy
Higher-energy beams
have greater
penetrating power and
thus deliver a higher
PDD

36. Fig. 4.3 Central axis depth dose distribution for different quality photon beams
3.2.1.1.1 Effect on Beam Quality & Depth

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38. EXERCISE
1.Draw an Isodose curve
2. Draw the PDD curves for electron
beam of energy 6 & 18 MeV and
photon beam of energy 6 & 18 MV
in water at 10 x 10 cm2 field size
and 100 cm SSD
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42. PDD of Photon beam
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PDD of Electron beam

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3.2.1.1.2 Effect on Field Size & Shape

44. Field size
 One of the most important parameters in treatment planning
 PDD increases as field size increases
 Field size dependence of PDD is less pronounced for higher energy
than for lower energies
 Field size smaller than 6 cm
Relative large penumbra region
Bell shape
TPS should be mandatory for small field size

45. 5*5 cm2 10*10 cm2 15*15cm2

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47. 3.2.1.1.2 Effect on Field Size & Shape
• the projection, on a plane
perpendicular to the beam axis, of
the distal end of the collimator as
seen from the front center of the
source
1. Geometrical field size
• the distance intercepted by a given
isodose curve (usually 50% isodose )
on a plane perpendicular to the
beam axis
2.Dosimetric ( Physical )
field size

48. Dd
Dmax
Scatter dose
3.2.1.1.2 Effect on Field Size & Shape
As the field size is
increased, the
contribution of the
scattered radiation to
the absorbed dose
increases
This increase in
scattered dose is
greater at larger
than at the depth of D
max , the percent
depth dose increases
with increasing field

49. 3.2.1.1.2 Effect on Field Size & Shape
Depends on
beam quality
1. The scattering probability or
cross-section decreases with
energy increase
2. the higher-energy photons are
scattered more predominantly in the
forward direction
3. the field size dependence of PDD is
less pronounced for the higher-energy
than for the lower-energy beams

50. 3.2.1.1.2 Effect on Field Size & Shape
PDD data for radiotherapy beams are usually
tabulated for SQUARE FIELDS
In clinical practice require rectangular and
irregularly shaped fields
A system of equating square fields to
different field shapes is required:
EQUIVALENT SQUARE

51. 3.2.1.1.2 Effect on Field Size & Shape
)(2 ba
ba
P
A



4
a
P
A
 P
A
a  4
QUICK CALCULATION OF THE EQUIVALENT FIELD PARAMETERS (P):

52. a
b
P
A
4
P
A
4
3.2.1.1.2 Effect on Field Size & Shape

53. •Change this rectangular field to the equivalent square.
10 x 20 cm field?
* See reference Table 9.2 in Faiz Khan’s book “The physics of radiation
therapy”
EXERCISE
Wedge factor
3.2.1.1.2 Effect on Field Size & Shape

54. Equivalent square Table
Part VIII.3.5 Determination of Dose to a Patient-I Slide 54
3.2.1.1.2 Effect on Field Size & Shape

55. •Equivalent circle has the same area as the equivalent
square
P
A
r 

4
a
b
P
A
4
P
A
4
r
3.2.1.1.2 Effect on Field Size & Shape

56. Square Field to Rectangular fields..
Part VIII.3.5 Determination of Dose to a Patient-I Slide 56
10cm
10cm
D
Measured data for this But may treat with this
20cm
5cm
Square field Rectangular field
3.2.1.1.2 Effect on Field Size & Shape

57. Equivalent square field
Part VIII.3.5 Determination of Dose to a Patient-I Slide 57
3.2.1.1.2 Effect on Field Size & Shape

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3.2.1.1.3 Effect on SSD

59. 3.2.1.1.3 Effect on SSD
Photon fluence
emitted by a
point source of
radiation varies
inversely as a
square of the
distance from
the source
The actual dose
rate at a point
decreases with
increase in
distance from
the source, the
PDD, which is a
relative dose,
increases with
SSD
MAYNEORD F
FACTOR

60. SSD affects the PDD and the depth of the isodose curves
PDD icreases with SSD
3.2.1.1.3 Effect on SSD

61. Fig. 4.5 Plot of relative dose rate as inverse square law function of distance from a point source.
Reference distance = 80 cm
f1+dm
f2+dm
f1+d
f2+d
f1+d f2+d
100
max

D
D
P d
3.2.1.1.3 Effect on SSD

62. 3.2.1.1.3 Effect on SSD
Under extreme
conditions such as lower
energy, large field (the
proportion of scattered
radiation is relatively
greater), large depth,
large SSD, the Mayneord
F factor has significant
errors
In general, the
Mayneord F
factor
overestimates
the increase in
PDD with
increase in SSD

63. d
dm
f2
d
dm
f1
r r
Mayneord F factor (for small fields since the scattering is minimal)
2
2
1
2
1
2

















df
df
df
df
F
m
m
Wedge factor

64. • Let P(d,r,f) be the percent depth dose at depth d for SSD=f
and a field size r
• The ratio of PDD at SSD=f2 to PDD at SSD=f1 is:
 
2
2
1
2
1
2
1
2
),,(
,,

















df
df
df
df
frdP
frdP
m
m
  F
frdP
frdP

),,(
,,
1
2
3.2.1.1.3 Effect on SSD

65. Depth dose - point to ponder
Depth dose increases with SSD - Why?
•Ans: Larger the SSD, smaller is the decrease
dose due to inverse square law
Part VIII.3.5 Determination of Dose to a Patient-I Slide 65

66. DepthDose Pointsto Remember
• In case of depth dose measurements the field size is defined at surface
of phantom
• Increases with SSD, Beam quality (energy), Field size.
• Decreases with depth
• Value is normalized to 100 % at depth of dmax for all field sizes
Part VIII.3.5 Determination of Dose to a Patient-I Slide 66

67. EXERCISE
For Co-60 beam, PDD for a 15x15 field size, 10 cm depth and
80 cm SSD is 58.4.
Find the PDD for the same field size and depth for a 100 cm
SSD.
Assume dm = 0.5 cm
3.2.1.1.3 Effect on SSD

68. ANSWER
F=1.043
P(10,15,100)
--------------- = 1.043
P(10,15,80)
Therefore desired depth dose
= 58.4 x 1.043 = 60.9
3.2.1.1.3 Effect on SSD