a system of dosimetric calculations - …phy428-528.ahepl.org/dose_pdd_tar2.pdf–the dependence of...

A SYSTEM OF DOSIMETRIC

CALCULATIONS

• Dose calculation based on PDD and TAR have Limitations– The dependence of PDD on SSD

– Not suitable for isocentric techniques

– TAR and SAR does not depend on SSD but as beam energy increases, the size of the chamber build-up cap for in-air measurements has to be increased and it becomes increasingly difficult to calculate the dose in free space from such measurements

• To overcome the limitations– Tissue-phantom ratio (TPR) and tissue

maximum ratio (TMR) are defined

INTRODUCTION

• The dose to a point in a medium may be

divided into primary and scattered

components.

• effective primary dose

– the dose due to the primary photons + those

scattered from the collimating system

• The scattered dose has two components

– collimator and phantom components

DOSE CALCULATION PARAMETERS

Collimator Scatter Factor (Sc)

• As the field size is increased,

the output increases because of

the increased collimator scatter

• Sc is commonly called the

output factor

• Defined as ratio of output

in air for a given field to

the output for reference

field

Phantom Scatter Factor (Sp)

• Sp account the change in scatter radiation

originating in the phantom at a reference depth as

the field size is changed.

• Sp is related to the changes in the volume of the

phantom irradiated for a fixed collimator opening

• Sp and Sc,p are defined at the reference depth of Dm

Phantom Scatter Factor (Sp)

Sc and Sp are actually field size correction factors

Tissue-Phantom and Tissue-Maximum Ratios

(Depth Correction Factors)

• The TPR is defined as the ratio

of the dose at a given point in

phantom to the dose at the same

point at a fixed reference depth,

usually 5 cm

• TPR(rd, d) =

D(rd, d) / D(rd, t0)

If t0 = dmax then TPR is called

TMR

• dmax should be chosen for the

smallest field and the largest

SSD.

Properties of TMR

• Independent of the

divergence of the beam

means independent of

SSD

– single table of TMRs

can be used for all SSDs

• Depends only on the

field size at the point

and the depth of the

overlying tissue.

Properties of TMR

• TMR and percent depth dose

f

t0

Scatter-Maximum Ratio

• Designed specifically for the calculation of

scattered dose in a medium

PRACTICAL APPLICATIONS

• a calculation system must be generally

applicable to the clinical practices, with

acceptable accuracy and simplicity for

routine use.

Accelerator Calculations-SSD

• PDD is a suitable quantity for calculations for

SSD setups

• Machines calibration

– deliver 1 cGy / MU at the reference depth t0 , for a

reference field size 10 x 10 cm and a source-to-

calibration point distance of SCD

– Sc is defined at the SAD, Sp relates to the field

irradiating the patient.

Accelerator Calculations-SSD

Accelerator Calculations-lsocentric Technique

• Unit calibrated to give 1 cGy / MU at the

reference depth to, calibration distance SCD, and

for the reference field (10 x 10 cm)

Accelerator Calculations-lsocentric Technique

Accelerator Calculations- Irregular Fields

• A Clarkson type integration may be performed to

give averaged SMR(d, rd) for the irregular field rd

• Above equation is valid only

for points along the central

axis of am beam that is

normally incident on an

infinite phantom with flat

surface

Where ri is the radius of ith sector and n is the total number of sectors

Accelerator Calculations- Irregular Fields• For off-axis points in a beam with nonuniform primary

dose profile. where Kp is the off-axis ratio

• PDD from TMR

Accelerator Calculations- Irregular Fields

• SSD Variation Within the Field

– g be the vertical gap distance, i.e., "gap" between skin

surface over Q and the nominal SSD plane

– The percent depth dose at Q is normalized with

respect to the Dm, on the central axis at depth to

Accelerator Calculations - Asymmetric Fields

• Jaw moved independently

• Allow asymmetric fields with field centers

positioned away from the true central axis of the

beam

• Sc , will depend on the actual collimator opening

– symmetric field of the same collimator opening as that

of the given asymmetric field

• Sp, can also be assumed to be the same for an

asymmetric field as for a symmetric field of the

same dimension and shape


• The primary dose distribution has been shown to vary

with lateral distance from central axis because of the

change in beam quality

• The PDD or TMR distribution along the central ray

of an asymmetric field is not the same as along the

central axis of a symmetric field of the same size and

shape

• the incident primary beam fluence at off-axis points

varies as a function of distance from the central axis,

depending on the flattening filter design


• beam flatness within the central 80% of the

maximum field size is specified within ±3% at a 10-

cm depth, ignoring off-axis dose correction in

asymmetric fields will introduce errors of that

magnitude under these conditions


• For SSD type

• For isocentric type

– where OARd(x) is the primary off-axis ratio at

depth d

OTHER PRACTICAL METHODS

• Irregular Fields


• Point Off-Axis– Q is off-axis point where dose is to be calculated and KQ is off axis ratio


• This off-axis decrease in dose is due to the

reduced scatter at point Q compared with point P


• Point Outside the Field


• Point Under the Block

– A patient is treated with a split field of overall size 15 x

15 cm, blocked in the middle to shield a region of size 4

x 15 cm on the surface

– given Co-60 beam, SSD = 80 cm, dose rate free space

for a 15 x 15-cm field at 80.5 cm = 120 rad/min, lead

block thickness = 5 cm with primary beam transmission

of 5%, and shadow tray (or block tray) transmission =

0.97



– (a) the treatment time to

deliver 200 cGy (rad) at a

10-cm depth at point Pin the

open portion of the field

– (b) what percentage of that

dose is received at point Q

in the middle of the blocked

area,

a system of dosimetric calculations - …phy428-528.ahepl.org/dose_pdd_tar2.pdf–the dependence of...

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