Dose calculation in brachytherapy for a 192Ir source using a primary and scatter dose separation technique

A dose calculation algorithm for brachytherapy is presented that reduces errors in absolute dose calculation and facilitates new techniques for modelling heterogeneity effects from tissues, internal shields and superficially positioned sources. The algorithm is based on Monte Carlo simulations for specific source and applicator combinations. The dose is scored separately, in absolute units, for the primary and different categories of scatter according to the photon scatter generation. Radial dose distributions for the primary dose and the total scatter dose are parametrized using functions based on simple one-dimensional transport theory. The fitted radial parameters are functions of the angle to the long axis of the source to account for the anisotropy of the dose distribution. The kerma in air at the reference point 1 m from the source is also simulated using Monte Carlo techniques and both the dose and kerma are normalized per source emitted radiant energy. The calculated kerma per radiant energy is used together with the measured reference air kerma rate and the ratio of the dose to the kerma to calibrate the calculated absolute dose rate. Data are presented for an 192Ir cylindrical source, in combination with water, nylon and stainless steel applicators. Values of the radial dose profiles, specific dose rate constants and corrections to the air kerma for attenuation and scatter in air are calculated. Anisotropy functions for the 192Ir source and a water-equivalent applicator are compared to published values. The effects of the applicator wall material on the radial dose distribution are also discussed.