Domain-Division Monte Carlo Dose Calculation Method for Particle Therapy

We present a Monte Carlo method using domain division, which enables dose calculations with huge voxel data under memory constraint. In this domain-division Monte Carlo (DDMC) method, we divide the entire geometry into several domains whose data size is such that Monte-Carlo simulations can be performed with a given amount of memory. We first perform a simulation for the domain to which the particle beam is incident and dump the informa- tion of exiting particles to a file. Then we perform simulations for the neighboring domains using the dump file as a source and, again, dump the information of exiting particles. We have developed and parallelized an external module to the Monte Carlo particle and heavy ion transport code PHITS that tallies the dose by repeating these procedures. The increase in calculation time due to dumping and re-reading is typically smaller than 10%, which is sufficiently small. We have also obtained good weak scaling up to 1,024 processors so far. The method we have developed is ex- pected to open a way to the realization of large-scale Monte Carlo dose calculations on general-purpose supercomputers and in a grid environment, where only a limited amount of memory is available, instead of on specia- lized large-memory machines.