Anisotropy functions for low energy interstitial brachytherapy sources:: an EGS4 Monte Carlo study

Anisotropy functions for low energy interstitial brachytherapy sources are examined. Absolute dose rates around Pd-103 seed model 200 and I-125 seed models 6702 and 6711 have been estimated by means of the EGS4 Monte Carlo simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. Following the formalism developed by the Interstitial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, theta), have been calculated. Our Monte Carlo results were compared against a limited set of measured data selected from the literature and other Monte Carlo results. Binding corrections and phantom material selection have been found to have no influence on the anisotropy function. The accuracy of the geometrical source models used for the Monte Carlo calculations was validated against experimental measurements of in-air relative fluence at 100 cm from the source. More detailed knowledge about the geometrical design of Pd-103 Seed model 200 is needed in order to improve the agreement with experimentally measured in-air fluence. Values for in-air fluence of I-125 model 6702 are sensitive to source position within the inner seed cylinder. Excellent agreement between calculated and measured in-air Auence is found for I-125 model 6711. It was observed that using in-air relative fluence at 100 cm from the source to calculate the anisotropy function yields a less anisotropic dose distribution at distances close to the source than full Monte Carlo simulation, in contradiction with experimental data. Our results have estimated statistical uncertainties of 1%-3% at the la level within clinically relevant regions, but contain systematic uncertainties related to the assumed geometrical details.