Inclusion of more sources in a commercial brachytherapy Monte Carlo

Abstract: Brachytherapy is a cancer treatment method where a solid radioactive source is placed inside the body in order to target cancer cells with a high dose of radiation while minimizing damage to surrounding healthy tissue. The most common method for treatment planning in brachytherapy is the TG43 formalism where the whole patient geometry is treated as water. Using TG43 leads to inaccuracies when trying to model materials that differ from water, such as air, bone or metal implants. A method that can accurately model particle transport in different media is the Monte Carlo (MC) method. In order to reduce the computational work required to perform MC simulations, pre-simulated phase-space files can be used which contain information about the initial particles position, direction and energy. In this work, four different brachytherapy sources were simulated and analyzed using the MC simulation software egs_brachy which is a general purpose and open-source code base. The four sources are referred to as Flexisource, Bebig, VS2000 and Ytterbium. The main objective was to create 4D histograms of the phase-space files and find a binning that could provide a correct dose distribution while maintaining a small file size. The histograms were then used as input for the Monte Carlo dose engine in the treatment planning system RayStation®. Two types of validations of the histograms were performed. Firstly, the Monte Carlo engine was compared to TG43 in a water patient. Secondly, each of the four sources were simulated in egs_brachy placed both inside and outside of a slab of material surrounded by water. The materials were lung, bone and tungsten. The egs_brachy simulations were then imported to RayStation® and compared against the Monte Carlo engine. The results for the material slabs were similar for the four sources. A notable difference between the egs_brachy dose and the Monte Carlo engine could be seen on the interface of the material slabs. The tungsten case showed a bigger difference than the other two materials. Overall the results were considered promising.