dose calculation parameters

8/10/2019 Dose Calculation Parameters

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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




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




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




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



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



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



Percent Depth Dose

dmax

d

SSD

Why does the absorbed dose D diminish? – Attenuation

– Inverse square

– Scatter



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



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



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



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



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



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)



Percent Depth Dose

Problem: Find the equivalent square of a 10x20

cm2 field

Rectangular fields:

Square fields:



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



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



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



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



Tissue Air Ratio

d

SSD

r d

d

SSD

r d

Free spacePhantom



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



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




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




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



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



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



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



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




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




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




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



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



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



Tissue Maximum Ratio

d

SAD

r d DP

dm

SAD

r d DQ



Tissue Maximum Ratio

TMR for 0x0 field

TMR derived from TAR



TMR and PDD



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



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



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)



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

dose calculation parameters

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