Development of a simplified virtual source model from Varian Clinac iX phase space files for photon beam radiotherapy

Abstract

Virtual source models (VSMs) are Monte Carlo (MC) input sources that are derived from phase space files (PSFs) to address their issues on file size, read time and limited particles. A bin-based VSM method developed recently was simpler and allowed greater compatibility across MC platforms but was not configured to be applicable on field-defined PSFs. In this work, we developed and validated simplified VSMs for the 6-MV Varian Clinac iX PSFs using MATLAB R2023a and the MC framework Particle and Heavy Ion Transport Code System (PHITS) v3.31. PSFs for 4×4. 10×10, and 20×20-cm² fields were retrieved from the International Atomic Energy Agency (IAEA) phase space database for external beam radiotherapy. By getting the particle distribution through a binning process, the behavior of the particles was replicated for sampling. The VSM was validated by comparing their particle parameter distribution and dose distribution with the originating PSF. Then, it was compared with treatment planning system-generated dose distribution. The VSMs for the 4×4. 10×10, and 20×20-cm² fields were 77.9%, 87.9%, and 93.8% smaller than the PSFs and were read 79.7%, 78.2%, and 33.0% faster, respectively. They were also shown to be valid for the two smaller field sizes while the largest field size did not pass the parameter distribution test. Percent depth dose generally had good agreement with TPS dose while dose profiles agreed well mostly for the two smaller fields only. As such, future studies may extend this VSM study to larger field sizes which require more factors to consider.