Shielding assessment for multi-ion radiotherapy based on ion-specific dose-defined workloads

The expansion of heavy-ion radiotherapy toward multi-ion operation requires a reassessment of shielding design methods that have traditionally relied on particle-number-based workloads. In facilities employing multiple ion species, this approach becomes inconsistent because different ions require substantially different numbers of primary particles to deliver the same prescribed dose. In this study, neutron shielding characteristics for helium, carbon, oxygen, and neon ion beams were evaluated using dose-defined workloads that reflect actual clinical and operational practice. Shielding effectiveness was assessed per unit physical dose to represent quality assurance and commissioning activities, and per unit Relative Biological Effectiveness (RBE)-weighted dose to represent treatment-related workload, based on PHITS Monte Carlo simulations. Under physical-dose normalization, carbon consistently produced the largest neutron effective dose across all shielding configurations. Under RBE-weighted dose normalization, carbon generally remained the most conservative reference ion, although high-energy helium under metallic beam-loss target conditions approached the carbon reference in selected configurations. These findings demonstrate that shielding outcomes are strongly dependent on the dose quantity used to define workload and support the continued use of carbon as the primary reference ion for multi-ion shielding assessment, while indicating that high-energy helium may approach the carbon reference only under restricted treatment-related metallic-target conditions.