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Volume 13, Number 1, 2008

Monte Carlo simulation of TLD response function: Scattered radiation field application

Alireza Shirazi, Seied Rabie Mahdavi, Ali Khodadadee, Mostafa Ghaffory, Asghar Mesbahi

Summary:

Aim Thermoluminescent dosimeters (TLDs) have various applications in non-primary
beam dosimetry. Monte Carlo simulation of TLD response was done in low energy
beams to improve its clinical use in scattered beam dosimetry.
Materials/Methods TLD material made from LiF doped with Mg and Ti sized 3.1×3.1×1mm3 was used for experimental measurements as well as modelling by MCNP-4c Monte Carlo
simulation. TLDs were irradiated for different doses of beam qualities ranging
from 120, 180, 200, 250 to 300kVp x-rays generated from an orthovoltage machine
and 1.25MeV gamma rays from a Co-60 teletherapy unit at reference depth in a water phantom. The simulation conditions were the same as experimental conditions. The calibration factor, (CF)q, and its quality dependence factor, (F X Co), were defined as:
(CF)q = Calibration Dose/TL, (F XCo) = TL(X)/Dmed (X)
TL(X)/Dmed (Co)
Results The normalized values of measured quality dependence factors for different x-ray
beams were 1.28, 1.24, 1.16, 1.07 and 1.03 for different beam qualities, respectively.
Comparatively, the MCNP simulated fi ndings were 1.134, 1.96, 1.139, 1.052 and 1.034. The change of calibration factor with energy followed the equation CF=B0+B1E+B2E2+B3E3, where CF and E are calibration factor and energy (keV), respectively. B0, B1, B2, B3 are constants.
Conclusions Our findings showed signifi cant deviation of true dose value when TLDs are calibrated at different beam qualities. The greatest deviation was 19.9±2.1% in beam
quality of 120kVp. Obtaining a dose response curve may be helpful to calculate
the calibration factor with more precision.

Signature: Rep Pract Oncol Radiother, 2008; 13(1) : 23-28

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http://www.sciencedirect.com/science/journal/15071367/19/2