Benchmarking the particle and heavy ion transport code system (PHITS) against MCNPX data for accelerator-based boron neutron capture therapy

Purpose: To verify the computational accuracy of the Particle and Heavy Ion Transport code System (PHITS) for Accelerator-Based Boron Neutron Capture Therapy (AB-BNCT) by benchmarking two reference Beam Shaping Assembly (BSA) designs previously optimized using the Monte Carlo N-Particle eXtended (MCNPX) code. This is a critical step in safely translating BNCT from large-scale nuclear reactors to accessible, hospital-based accelerator facilities. Methods: Two MCNPX BSA models were reconstructed in PHITS and assessed to calculate four beam quality parameters: (1) epithermal flux, (2) thermal flux-epithermal flux ratio, (3) fast neutron dose-epithermal flux ratio, and (4) gamma dose rate-epithermal flux ratio. Values from PHITS were compared with the original MCNPX results and then assessed for compliance against the guidelines established by the International Atomic Energy Agency (IAEA). Results: Across both independent models, PHITS consistently overestimated epithermal flux (yielding 8.66×109 n/cm2∙s versus Fauzi’s 2.33 ×109 n/cm2∙s) while underestimating thermal flux (yielding 1.33×109 n/cm2∙s versus Dorostkar’s 4.16 ×109 n/cm2∙s). This spectral shift pattern is attributed to inherent differences in nuclear data libraries; PHITS utilizes updated JENDL-4.0 cross-sections, angular distributions, and thermal scattering data, whereas the reference MCNPX studies utilized older ENDF/B-VI evaluations. Notably, despite these spectral shifts, the PHITS simulations demonstrated that both reconstructed models successfully satisfied all IAEA therapeutic guidelines compared to the original MCNPX studies which exceeded fast dose limits. Conclusion: PHITS provides a highly capable Monte Carlo framework for modeling AB-BNCT beam shaping assemblies. The benchmarking highlights how updated JENDL-4.0 nuclear data libraries in PHITS yield distinct, clinically compliant spectral characteristics compared to MCNPX calculations. With this, the benchmarking allows us to proceed to the next step, which is the material and geometric optimization of novel BSA configurations.