Monte Carlo simulated correction factors for high dose rate brachytherapy postal dosimetry audit methodology

Abstract

Background and Purpose: Full-scatter conditions in water are impractical for postal dosimetry audits in brachytherapy. This work presents a method to obtain correction factors that account for deviations from full-scatter water-equivalent conditions for a small plastic phantom. Material and Methods: A 16 × 8 × 3 cm phantom (PMMA) with a radiophotoluminescent dosimeter (RPLD) at the centre and two catheters on either side was simulated using Monte Carlo (MC) to calculate correction factors accounting for the lack of scatter, non-water equivalence of the RPLD and phantom, source model and backscatter for HDR 60Co and 192Ir sources. Results: The correction factors for non-water equivalence, lack of full scatter, and the use of PMMA were 1.062 ± 0.013, 1.059 ± 0.008 and 0.993 ± 0.009 for 192Ir and 1.129 ± 0.005, 1.009 ± 0.005 and 1.005 ± 0.005 for 60Co respectively. Water-equivalent backscatter thickness of 5 cm was found to be adequate and increasing thickness of backscatter did not have an influence on the RPLD dose. The mean photon energy in the RPLD for four HDR 192Ir sources was 279 ± 2 keV in full scatter conditions and 295 ± 1 keV in the audit conditions. For 60Co source the corresponding mean energies were 989 ± 1 keV and 1022 ± 1 keV respectively. Conclusions: Correction factors were obtained through the MC simulations for conditions deviating from TG-43, including the amount of back scatter, and the optimum audit set up. Additionally, the influence of different source models on the correction factors was negligible and demonstrates their generic applicability.

Keywords

• Brachytherapy
• Monte Carlo simulations
• Dosimetry audit
• Reference Air Kerma Rate