Investigation of perturbation correction factors Investigation of perturbation correction factors for PTW semiflex 0.125 cm3 chamber with for PTW semiflex 0.125 cm3 chamber with

EGSnrc Monte Carlo transport code.EGSnrc Monte Carlo transport code.

ContributorsContributorsM. Anwarul IslamM. Anwarul Islam

SQUARE Hospitals Ltd, Dhakaanwar.amch@yahoo.com

&

G. A. ZakariaG. A. ZakariaGummersbach Hospital, Academic Teaching

Hospital of the University of Cologne, Germany

Objectives

To calculate the perturbation correction factors for a specific chamber

To estimate the statistical uncertainty

To compare the value calculated by Monte Carlo with theoretical/ experimental value

Spencer-AttixCavity theoryCavity theory

Absorbed-dose to water dosimetry uses the Spencer-Attix cavity theory to relate the absorbed dose to the gas in the ion chamber, Dgas, to the dose to the surrounding phantom medium, Dmed, by the following expression:

(1)ρLDD

med

gas

_

gasmed

=

is the ratio of the spectrum averaged restricted mass collision stopping power for the medium to that of the gas.

med

gas

_

ρL

Cont.Cont.

In case of radiotherapy dosimetry, In case of radiotherapy dosimetry, water phantom and air filled ionization water phantom and air filled ionization chamber normally be used. So, the chamber normally be used. So, the modified cavity theory as follows:modified cavity theory as follows:

(1)ρLDD

water

air

_

airwater

=

averaged restricted mass collision stopping power ratio

water

air

_

ρL

Cont.Cont.Spencer-Attix cavity theory has three Spencer-Attix cavity theory has three necessary assumptions:necessary assumptions:

(1)1) the cavity does not change the electron the cavity does not change the electron spectrum in the mediumspectrum in the medium

(2)(2) the dose in the cavity comes from the dose in the cavity comes from electrons that enter the cavity and not electrons that enter the cavity and not from from those that are created within the those that are created within the cavitycavity

(3)(3) charged particle equilibrium (CPE) will charged particle equilibrium (CPE) will be existbe exist

Unluckily, the ion chambers do not Unluckily, the ion chambers do not satisfy the above assumptionssatisfy the above assumptions

Why not satisfy the assumptions?

Presence of central electrode in ion chamber which is not equivalent material with air.

Need correction factor for central Need correction factor for central electrode, electrode, PPcelcel

Presence ion chamber wall which is differ from air.

Need correction factor for wall, Need correction factor for wall, PPwallwall

Cont.

Presence of chamber stem in the phantom

Need correction factor for stem, Need correction factor for stem, PPstemstem

Presence of air in ion chamber which is low dense material comparatively with water. Electron spectrum will be changed due to this air cavity in the phantom.

Need correction factor for spectrum Need correction factor for spectrum change, change, PPreplrepl

Cont. Finally, Spencer-Attix cavity theory with all over Finally, Spencer-Attix cavity theory with all over

perturbation factor followed by equation (1) isperturbation factor followed by equation (1) is

)2(replwallstemcel

water

air

_

airwater PPPPρLDD

=

Cont. The value of perturbation factors are

energy dependent

Each chamber should have separate correction factors for energy basis but it is quite impossible, expensive and time consuming.

PSDL / SSDL choice a specific energy (60Co) for individual chambers to measure chamber correction factor.

Need additional correction factor, KQ

Materials and Method

PTW semiflex 0.125 cm3 ionization chamber

Model 31010Wall material = PMMAWall material density = 1.19 g/cm3

Wall thickness = 0.55 mmCentral electrode = AluminumAluminum density = 2.69g/cm3

Cont.

Electrode diameter = 1.1 mmLength of electrode = 5 mmThickness of graphite coat = 0.15 mmGraphite density = 0.82g/cm3

Radius of sensitive volume=2.75 mmLength of sensitive volume = 6.5 mm

Cont.Graphite coat

0.69

0.5

0.55PMMA Electrode *

Air cavity

Fig.1: Schematic figure of a PTW semiflex 0.125 cm3 ion

chamber

Monte Carlo codes

EGSnrc Monte Carlo codes are used to calculate all factors

The EGSnrc codes introduced by National Research Council (NRC) of Canada

The NRC grants the user a non-transferable, non-exclusive license to use this system free of charge only for non-commercial research or educational purposes.

Cont.

60Co spectrum was used for all calculation

SSD was = 80 cm Water depth of calculation = 5 cm Field size 10 cm diameter 20cm×20cm×20cm water phantom

was used

Cont.

Photon cutoff energy was 0.001MeV Electron cutoff energy was 0.521MeV 109 particle histories were simulated for

each calculation XCOM Photon Cross Sections data are used

from NIST (published by Hubbell et al).

Calculation of Calculation of PPelecelec

Dose calculation to the chamber effective point of measurement with central electrode, Delec

Dose calculation on chamber effective point of measurement without central electrode, Dnoelec

PPelecelec = = DDnoelec noelec / / DDelecelec

A

PTW semiflex chamber with central electrodePTW semiflex chamber with central electrode

**

B

PTW semiflex chamber with no central electrodePTW semiflex chamber with no central electrode

**

Effective point of Effective point of measurementmeasurement

PPelecelec = = DDBB / / DDAA

Calculation of Calculation of PPelecelec

Calculation of Calculation of PPwallwall

Dose calculation on chamber effective point of measurement without central electrode with air filled, Dwall

Dose calculation on chamber effective point of measurement without central electrode and wall, Dnowall

The existing wall material replaced by water for Dnowall calculation

PPwallwall = = DDnowall nowall / / DDwallwall

Calculation of Calculation of PPwallwall

CC

Air filled cavityAir filled cavity

PTW semiflex chamber with only wall

Air filled cavityAir filled cavity

PTW semiflex chamber with only wall

**

Effective point of Effective point of measurementmeasurement

PPwallwall = = DDDD / / DDCC

DD

**

Effective point of Effective point of measurementmeasurement

Calculation of Calculation of PPreplrepl

Dose calculation on chamber effective point of measurement without central electrode and wall with water vapor filled, Dsteam

Dose calculation on chamber effective point of measurement without chamber in water medium at small voxel, Drepl

The existing chamber material replaced by water for Drepl calculation

PPreplrepl = = DDreplrepl / / DDstreamstream

Calculation of Calculation of PPreplrepl

Chamber cavity filled with water vapor

Water vaporWater vapor**

Effective point of Effective point of measurementmeasurement

E

**

Effective point of Effective point of measurementmeasurement

FCalculation of dose at small voxel in water

PPreplrepl = = DDFF / / DDEE

Results and DiscussionsResults and Discussions

The calculated value Pwall is found to be 1.008 ± 0.6%

This is in good agreement with the published value 1.001 in TRS-398 with PTW 31003 flexible ion chamber.

Cont.Cont.

The calculated value Pcel is found to be 0.995 ± 0.7%

This is in good agreement with the published value 0.993 in TRS-398 with PTW 31003 flexible ion chamber.

Cont.Cont.

The calculated value of Prepl is found to be 0.992 ± 0.4%

The AAPM’s TG-51 and TG-21 dosimetry protocols use a value of Prepl = 0.992 for a cylindrical chamber of inner diameter of 6 mm in a 60Co beam.

This value is from the work of Cunningham and Sontag who derived Prepl based on analytical calculations and experiments.

For the same quantity the IAEA’s TRS-398 Codes of Practice use a value of 0.988 which is based on the measured data of Johansson et al.