Influences of spherical phantom heterogeneities on dosimetric charactristics of miniature electronic brachytherapy X-ray sources: Monte Carlo study

Dose calculations in current brachytherapy treatment planning systems (TPS) are commonly based on TG-43U1 formalism. These TPS are obtained by superposition principle of single-source dosimetric parameters in liquid water, neglecting the effects of tissue heterogeneity. In this work, the sensitivity of the TG-43U1 based radial dose function (g(r)) of Miniature Electronic Brachytherapy X-ray Sources (MEBXS) to bone-heterogeneity was examined. To quantify the heterogeneity effects for g(r), a series of Monte Carlo (MC) based radiation transport simulations at the center of homogeneous and heterogeneous spherical phantoms were performed using the MCNP5 code. The ratio of the g(r) in the heterogeneius phantom to the uniform soft tisuue phantom as a function of the bone thickness was determined. These results indicated that for 40. keV beam, the maximum ratios for thicknesses of 1. cm and 2. cm were 3.36 and 3.27, respectively. These values changed to 4.28 and 4.06, for 60. keV beam, respectively. Introduction of 0.5. cm or 1. cm red marrow, into the interior of the cortical bone changed the maximum variations to, 3.54, and 3.57 for 40. keV, and 4.28, and 4.25, for 60. keV, respectively.