Monte Carlo-based dosimetric studies of a locally developedTm LDR brachytherapy seed source.

Tm is being explored as a source for applications in brachytherapy. Although it has adequate physical properties, such as a short half-life (128.6 d), high specific activity and a mean photon energy of about 66 keV, it has a drawback of low photon yield (only about six photon emissions/100 beta emissions). The objective of this work is to study the dosimetric characteristics of a locally developedTm brachytherapy seed source using the Monte Carlo-based EGSnrc code system. In this study, we calculate the dose rate constant, air-kerma strength, radial dose function, anisotropic function and 2D dose-rate distributions in water. Separate simulations are carried out by considering the photon (gamma and characteristic x-ray) and beta spectra of the source. For regions close to the source (surface of the source << 0.4 cm), the dose is solely due to direct dose deposition by beta particles. At larger distances (0.4 cm <<10 cm), the dose is due to bremsstrahlung photons produced by beta particles and photon emissions. The calculated value of the dose rate constant is 1.217 ± 0.052 cGy hU. The value ofper mCi is 0.029 ± 0.0009 U mCi. The contributions of the inherent photon emission and the bremsstrahlung photons to the totalare 0.58 and 0.42, respectively.