Efficient particle transport simulation through beam modulating devices for Monte Carlo treatment planning.

For Monte Carlo treatment planning it is essential to model efficiently patient dependent beam modifying devices, e.g., Multi-Leaf Collimators (MLC). Therefore a Monte Carlo geometry tracking procedure is presented allowing the simulation of photon and electron transport through these devices within short calculation time. The tracking procedure is based on elemental regions, on surfaces (mainly planes) to separate the regions as well as on bit patterns and bit masks to identify the regions. Photon cross sections for photoelectric absorption, Compton scattering and pair production as well as electron stopping powers and ranges are provided by the Physical Reference Data of the National Institute of Standards and Technology (NIST). The tracking procedure is implemented in c + + with object-oriented design based on c + + class hierarchies and inheritance. Using the geometry technique, several MLC models are constructed. Some of them take into account tongue-and-groove effects as well as curved leaf ends. The models are integrated into the Monte Carlo dose calculation engine XVMC for treatment planning. The system is tested by comparing different MLC implementations and by verification with measurement.