MONTE-CARLO CALCULATION OF KERMA TO A POINT IN THE VICINITY OF MEDIA INTERFACES

Recent work has demonstrated that Monte Carlo photon transport (MCPT) simulation is a practical and accurate dosimetry tool for characterizi ng brachytherapy dose distributions in the presence of complex three-d imensional (3D) geometries. Because of the steep dose gradients encoun tered in this application, the choice of estimation algorithm, i.e.. t he algorithm for extracting dose estimates from individual photon hist ories, is critical to ensure adequate spatial resolution and computati onal efficiency. While conventional MCPT dose estimation techniques ar e sufficiently accurate in many cases, they inevitably fail when the p oint of interest is located near media interfaces, since they require the point of interest to be contained by a region of homogeneous mediu m of several millimetres in diameter, This severely limits the domain of application of MCPT in brachytherapy dosimetry and prevents dose es timation at points such as those immediately behind lead shields conta ined in gynaecological applicators, or simulation of small intricately structured detector response. We have successfully adapted to an MCPT code a neutron-flux estimator, the once-more-collided flux estimator (OMCFE), which overcomes this problem. OMCFE is derived from the famil iar next-flight estimation algorithm, and, through the use of a photon trajectory resampling scheme. does not require homogeneous media at t he point of interest. It is therefore a true point -dose estimator. OM CFE was tested against a variety of one- and three-dimensional benchma rk problems. These benchmarks included mono-energetic point sources of 30 and 600 keV, as well as a 3M model 6702 I-125 seed, a Cs-137 tube, and an Ir-192 seed in geometries approximating common brachytherapy d osimetric problems. OMCFE agrees closely with conventional MCPT dose e stimation algorithms in solving these benchmark problems, while in man y cases being much more efficient. It therefore makes possible brachyt herapy dose estimation with high spatial resolution, in contrast to th e larger than 1 mm spatial resolution achievable using conventional MC PT estimation algorithms. It should be noted that the method presented in this paper is only valid when charged-particle equilibrium is assu med.