Neutrons absorbed dose rate calculations for interstitial brachytherapy with 252Cf sources

The AAPM TG-43 protocol was used to evaluate the neutron absorbed dose rate from 252Cf sources for two tissue substitute materials, five normal tissues and six malignant tumours. Three 252Cf source models (AT, VariSource and μSelectron) were simulated using the Monte Carlo MCNPX code in spherical geometry for obtaining the following factors: (a) conversion coefficient of neutron air kerma strength, (b) neutron dose rate constant, (c) neutron radial dose function, (d) geometry function, (e) anisotropy function and (f) neutron absorbed dose rate. For the 252Cf AT source and a Watt fission spectrum, the calculated reference neutron dose rate in water was D ˙ N ( r 0 , θ 0 ) =1.916 cGy h−1 μg−1. The results for the reference neutron dose rate in normal tissues and malignant tumours show how small variations in their elemental composition produce variations in the neutron absorbed dose rate up to 14.9% and 8.4%, respectively, when water and muscle are the reference medium.