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Field Size, SSD and SAD
Field size of the radiation beam is defined by
collimators
Field size changes with distance from the sourceof radiation because of divergence
Field size may be specified either geometrically
or dosimetrically
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Field Size, SSD and SAD
Geometric field size
– Is defined as the projection of the distal end of
the collimator onto a plane perpendicular to thecentral axis at source-surface-distance (SSD) or
source-axis (isocenter)-distance (SAD)
– Corresponds to the field defined by the light
localizer
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Field Size, SSD and SAD
Dosimetric field size
– Is the distance intercepted by 50% isodose on a
plane perpendicular to the beam axis at areference distance from the source
– One should check that the geometric and
dosimetric field sizes are equal
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Field Size, SSD and SAD
Each isocentric machine has its own SAD, which
for most modern linear accelerators is 100 cm.
When the gantry rotates around the patient, theSSD will continuously change
However, the source and isocenter are at a fixed
distance and therefore the SAD does not change
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SSD technique
- source to surface distance = constant
- source to detector distance = variable
- field size defined at the surface
- uses PDD
SAD technique
- source to surface distance = variable
- source to axis distance = constant
- field size defined at the isocenter
- uses TMR
Diagram illustrating the meaning of SSD,
SAD and field size
d
SSD
d
SADField
size
Field
size
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Percent Depth Dose
Percent depth dose (PDD) is defined in central
axis (CAX) and SSD setup
PDD is the ratio, expressed as a percentage, ofthe absorbed dose at a given depth d to the
maximum absorbed dose at dmax
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Percent Depth Dose
dmax
d
SSD
Why does the absorbed dose D diminish? – Attenuation
– Inverse square
– Scatter
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Percent Depth Dose
PDD increases with beam energy (at depths
beyond dmax)
– Higher-energy beams have greater penetrating
power, thus delivering a higher percentage dosePDD decreases with depth beyond dmax
For depths smaller then dmax there is an initial build-
up of dose
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Percent Depth Dose
Build-up region leads to skin-sparing effect.
– Is the region between the surface and the point of
maximum dose
– Becomes more pronounced as the energy isincreased (the point of maximum dose lies
deeper into the tissue)
The region beyond the buildup region is called
the region of electronic equilibrium where asmany electrons stop in any volume as are set in
motion in it
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Percent Depth Dose
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
Depth in phantom, (cm)
P e r c e n t D e p t h
D o
s e
Buildup Region
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Percent Depth Dose
PDD increases with field size
– Contribution of the scattered radiation to the absorbed
dose increases as the field size increases; since this
increase in scattered dose dose is greater at larger
depths than at the depth dmax, PDD increases with
increasing field size
– The field size dependence of percent depth dose isless pronounced for higher energy than for lower-
energy beams
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Percent Depth Dose
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
Depth in Phantom, (cm)
P e r c e n t D e p t h
D o s e
40 x 40 cm
10 x 10 cm
3 x 3 cm
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Percent Depth Dose
Percent depth dose data is tabulated for squarefields
Not all fields in radiotherapy are square!
Sterling’s approx: Equivalent square technique
A rectangular field is equivalent to a squarefield if they have the same area/perimeter
(A/P)
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Percent Depth Dose
Problem: Find the equivalent square of a 10x20
cm2 field
Rectangular fields:
Square fields:
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Percent Depth Dose
Percent depth dose increases with increasing
SSD
– Because of the effects of the inverse square law
For treatment of deep-seated lesions withmegavoltage beams, the minimum
recommended SSD is 80 cm
Percent depth doses are measured at standard
SSD (80 or 100 cm)
In a clinical situation the SSD set on a patient
may be different from standard SSD
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Percent Depth Dose
To correct for this one uses Mayneord F factor
Where:
Mayneord F factor overestimates the increase in
PDD with increase in SSD
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Percent Depth Dose
Problem: The PDD for a 15x15 cm2 field size , 10
cm depth, and 80 cm SSD is 58.4 (60Co beam).
Find PDD for the same field size and depth for a
100 cm SSD
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Tissue Air Ratio
Tissue air ratio (TAR) has been defined toremove the SSD dependence (TARindependent of SSD)
TAR increases with: – Energy increasing
– Field size increasing
– Depth decreasing
TAR is the ratio of the dose (Dd) at a given pointin the phantom to the dose in free space (Dfs) atthe same point
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Tissue Air Ratio
d
SSD
r d
d
SSD
r d
Free spacePhantom
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Backscatter Factor
Backscatter factor (BSF) is the TAR at the
depth of maximum dose on central axis of the
beam
BSF increases with field size About 8 MV, the scatter at the depth of Dmax
becomes negligible small and the BSF approaches
its minimum value unity
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Calculation of Dose in Rotation Therapy
TAR is most useful for calculations involving isocentrictechniques of irradiation
Rotation or arc therapy is a type of isocentric irradiationIn which the source moves continuously around the axisof rotation
One has to determine the average TAR at the center – Draw the contour of the patient in a plane containing the
axis of rotation
– Place the isocenter within the contour (usually in themiddle of the tumor
– Draw radii from this point at selected angular intervals – Each radius represents a depth for which TAR can be
obtain from TAR table for a given beam energy and fieldsize defined at the isocenter
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Calculation of Dose in Rotation Therapy
Angle Depth
along
Radius
TAR
0 16.6 0.444
30 14.6 0.499
60 9.0 0.69190 14.0 0.515
120 15.6 0.470
150 16.2 0.450
180 16.2 0.450
210 14.6 0.499
240 11.2 0.606
270 11.0 0.614
300 14.2 0.507
330 16.0 0.456
60Co beam, field size @isocenter
6x6 cm2
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Calculation of Dose in Rotation Therapy
Example
Using the TARavg determined previously, determine
the treatment time to deliver 200 cGy at the center
of rotation, given data: dose rate free space for 6x6
cm2 60Co at SAD is 86.5 cGy/min
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Scatter Air Ratio
Scatter air ratio (SAR) is used to calculatescattered dose in a medium
The computation of the primary and scattered
dose separately is particularly useful in thedosimetry of irregular fields
SAR by definition is the ratio of the scattereddose at a given point in the phantom to the dose
in free space at the same pointSAR depends on beam energy, depth and fieldsize, but is independent on SSD
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Scatter Air Ratio
SAR can be derived from TAR at a given fieldand the TAR for 0x0 field
TAR(d,0) represents the primary component ofthe beam
SARs are primarily used in calculating scatter in
a field of any shapeSARs are tabulated as functions of depth radiusof a circular field at that depth
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Collimator Scatter Factor
Collimator scatter factor (Sc) is the ratio of the
output in air for a given field to that for a
reference field (e.g.10x10 cm2)
Sc is measured with an ion chamber with abuildup cap
Sc normally is measured at SAD
The collimated field size is always specified atSAD
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Phantom Scatter Factor
Phantom scatter factor (Sp) takes into account
the change in scatter radiation originating in the
phantom at a reference depth as the field size is
changedSp is the ratio of the dose rate for a given field at
depth of maximum dose dm to the dose rate at
the same depth for the reference field (e.g.
10x10 cm2) with the same collimator opening
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Phantom Scatter Factor
A more practical way of measuring Sp
Sc,p(r) is the total scatter factor (total output
factor) defined as the dose rate at a reference
depth for a given field size r divided by the
dose rate at the same point and depth for thereference field size (10x10 cm2)
Sc,p(r) contains both the collimator and
phantom scatter
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Phantom Scatter Factor
If a part of the beam is blocked, the field used for
Sc is different from the field use for Sp
One will determine an effective field size
The blocks used for field shaping are placed on a
tray. To correct for this one has to add a tray factorTF
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Phantom Scatter Factor
Example
The unblocked field defined at SAD is 4x10 cm2. During
the treatment one will blocked 30% of this field. Find the
effective field size
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Arrangement for Measuring Sc and Sc,p
SAD
AirReference
field
SAD
Phantom
Reference
field
dm
Arrangement for measuring Sc
- Chamber with a buildup cap
- Determine the output in air for a given
field size to that for a reference field
Arrangement for measuring Sc,p
-Chamber without a buildup cap
- Determine the output in phantom at
the reference depth for a given field
size to that for a reference field
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Tissue Phantom and Tissue Maximum
Ratios
Tisssue phantom ratio (TPR) is the ratio of thedose at a given point in a phantom to the doseat the same point at a fixed reference depth,usually 5 cm
Tissue maximum ratio (TMR) is TPR whenreference depth is the depth of maximum dosedm
TMR and TPR are defined in a CAX SAD setup.For TMR calculations consider only attenuationno inverse square
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Tissue Maximum Ratio
d
SAD
r d DP
dm
SAD
r d DQ
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Tissue Maximum Ratio
TMR for 0x0 field
TMR derived from TAR
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TMR and PDD
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Scatter Maximum Ratio
Scatter maximum ratio (SMR) is the ratio of the
scattered dose at a given point in phantom to the
effective primary dose at the same point at the
reference depth of maximum dose
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Off Center Factor
Off center factor (OCF) is the ratio of dose at off-axis
point of interest to the dose at the central axis at the
same depth for a symmetrically wide open field
d
DP DQ
x
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Wedge Factor
Special filters are placed in the path of the beam
to modify its dose distribution
Wedge filter is a wedge-shaped absorber which
causes a progressive decrease in the intensityacross the beam, resulting in a tilt of the isodose
curves from their normal positions
To correct for this one uses a wedge factor (WF)
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Dose Calculations
SSD setup
SAD setup
k (=1.0 cGy) is the output determined with SAD
or SSD calibration
It doesn’t matter where one calculates theoutput. What is important is to know where it
was. One can change from one setup to another
using ISC