Experimental and computational verifications of the dose calculation accuracy of PHITS for high-energy photon beam therapy

This study aimed to assess the accuracy of the particle and heavy-ion transport code system (PHITS) for its application of clinical dosimetry in high-energy photon beam therapy. Percentage depth dose (PDD) and beam profiles in a water phantom with various field sizes created by a Clinac 21EX linear accelerator were measured using an ionization chamber. Experimental setups were precisely reproduced by PHITS version 3.24, and the relative percent differences (%Diff) between the measured and calculated data were evaluated. Furthermore, the PHITS results were compared with the corresponding data obtained from the Electron Gamma Shower National Research Council (EGSnrc), including the energy spectra. The average %Diffs obtained from the PHITS and measurements were less than a few percent in most cases, which are lower than the tolerance levels recommended by European Society for Radiotherapy and Oncology Booklet. In the penumbra region of beam profiles, the differences between the calculated and measured distances from the central axis to the 50% dose level were less than 2 mm, which are also within the tolerance level. The %Diffs between the PHITS and EGSnrc were within ± 1% in most cases. These results demonstrate the potential clinical application of the PHITS in high-energy photon beam therapy.