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, [email protected]
&
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.