Determining TG-43 brachytherapy dosimetry parameters and dose distribution for a 131Cs source model CS-1

Theoretical and experimental dosimetric studies have been supplied useful information on the dependence of the brachytherapy source geometry and materials (1-5). Usually, Monte Carlo method is used to define dose distribution function, the radial dose variation, and the dose calculation close to the source in brachytherapy. Chen et al. (6). calculated the distribution of absorbed dose around commercially available 131Cs seeds model Cs1 by Monte Carlo simulation. 131Cs has a higher average energy than any other commonly used prostate brachytherapy isotope in the market such as 103Pd and 125I. Energy is a key factor in how uniformly the radiation dose can be delivered throughout the prostate. Also, 131Cs has the shortest halflife of any prostate brachytherapy isotope at 9.7 days. 131Cs delivers 90% of the prescribed dose to the prostate in just 33 days compared to 58 days for 103Pd and 204 days for 125I. The short half-life of 131Cs reduces the duration of time during which the prostate receives the irritating effects of the radiation. Another benefit to the short half-life of 131Cs is what is known as the biological effective dose (BED). BED is a way for providers to predict how an isotope will perform against slow versus fast growing tumors. Studies have shown 131Cs is able to deliver a higher BED across a wide range of tumor types than either 125I or 103Pd. Although prostate cancer is typically viewed as a slow growing cancer it can present with aggressive features. 131Cs's higher BED may be particularly beneficial in such situations. Currently, the IsoRay 131Cs seed is used exclusively for the treatment of prostate cancer (7-12). For calculating dose distribution and TG-43 brachytherapy parameters usually Monte Carlo codes as MCNP, EGS4, GEANT4 are applied. In this present work, we have used MCNP4C code (12) to calculate relative dose in the phantom.