Array

Abstract

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_C^X_o), were defined as: [[mml:math altimg="si1.gif"]] [[mml:mrow]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:mi]]C[[/mml:mi]] [[mml:mi]]F[[/mml:mi]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[mml:mi]]q[[/mml:mi]] [[mml:mi]] [[/mml:mi]] [[mml:mo]]=[[/mml:mo]] [[mml:mi]] [[/mml:mi]] [[mml:mtext]]Calibration[[/mml:mtext]] [[mml:mi]] [[/mml:mi]] [[mml:mtext]]Dose[[/mml:mtext]] [[mml:mo]]/[[/mml:mo]] [[mml:mi]]T[[/mml:mi]] [[mml:mi]]L[[/mml:mi]] [[mml:mo]],[[/mml:mo]] [[mml:mi]] [[/mml:mi]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:msubsup]] [[mml:mi]]F[[/mml:mi]] [[mml:mrow]] [[mml:mi]]c[[/mml:mi]] [[mml:mi]]o[[/mml:mi]] [[/mml:mrow]] [[mml:mi]]X[[/mml:mi]] [[/mml:msubsup]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[mml:mi]] [[/mml:mi]] [[mml:mo]]=[[/mml:mo]] [[mml:mfrac]] [[mml:mrow]] [[mml:mi]]T[[/mml:mi]] [[mml:mi]]L[[/mml:mi]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:mi]]X[[/mml:mi]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[mml:mo]]/[[/mml:mo]] [[mml:msub]] [[mml:mi]]D[[/mml:mi]] [[mml:mrow]] [[mml:mi]]m[[/mml:mi]] [[mml:mi]]e[[/mml:mi]] [[mml:mi]]d[[/mml:mi]] [[/mml:mrow]] [[/mml:msub]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:mi]]X[[/mml:mi]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[/mml:mrow]] [[mml:mrow]] [[mml:mi]]T[[/mml:mi]] [[mml:mi]]L[[/mml:mi]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:mi]]X[[/mml:mi]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[mml:mo]]/[[/mml:mo]] [[mml:msub]] [[mml:mi]]D[[/mml:mi]] [[mml:mrow]] [[mml:mi]]m[[/mml:mi]] [[mml:mi]]e[[/mml:mi]] [[mml:mi]]d[[/mml:mi]] [[/mml:mrow]] [[/mml:msub]] [[mml:mo stretchy="false"]]([[/mml:mo]] [[mml:mi]]C[[/mml:mi]] [[mml:mi]]o[[/mml:mi]] [[mml:mo stretchy="false"]])[[/mml:mo]] [[/mml:mrow]] [[/mml:mfrac]] [[/mml:mrow]] [[/mml:math]]

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 findings 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 significant 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.