Dosimetric effects of composition, location and size of tissue heterogeneities on 252Cf neutron brachytherapy

The purpose of this study is to evaluate the effect of tissue heterogeneities on dose distribution in Californium-252 (Cf-252) neutron brachytherapy. The effect of location and size of heterogeneity on dose distribution was also evaluated. Neutron and photon dose rate distributions were determined in a water phantom in presence of air, lung, soft tissue and bone heterogeneities using MCNPX code. To benchmark the Monte Carlo simulation of the Cf-252 source, air kerma strength (S-KN), dose rate constant (A(N)) and radial dose function (g(N)(r)) were calculated and compared with previously reported data. Results showed a considerable reduction of neutron dose rate (up to 66%) inside heterogeneities, especially in air and bone heterogeneities, while the reduction of total photon dose rate was found less significant (up to 10%). In the presence of a heterogeneity, dose rate, fluence and energy spectrum were significantly different with respect to the homogenous phantom. The contribution of photon dose to the total dose in the presence of air and bone was dominant, compared to the neutron dose, whereas this photon contribution was reduced after passing the heterogeneity. As the bone heterogeneity size was increased from 1 x 1 x 1 cm(3) to 1 x 3 x 1 cm(3), the total dose and neutron energy fluence decreased of about 50% and 70%, respectively.