Radiation Protection and Shielding Assessment of a Gantry-Less Proton Therapy System Retrofit into a Linac Vault
Abstract
Purpose
With the advent of pencil beam scanning in proton therapy, the necessity of a gantry has become less compelling. More broadly, gantry-less proton therapy systems offer a potential pathway to reduce system complexity, footprint, and cost, thereby improving patient access. We propose a novel gantry-less proton therapy design utilizing a synchrotron-based beamline and evaluate secondary radiation shielding requirements intended for installation within a conventional LINAC vault.
Methods
A compact gantry-less proton therapy beamline was modeled using Monte Carlo simulation with geometry, vault layout, and shielding based on clinical LINAC specifications. Neutron ambient dose equivalents were scored inside and outside the room and compared with ICRP and NCRP limits. Validation against radiation survey measurements at the Gordon Browne Proton Therapy Center showed agreement within 37-52% for 250-MeV proton beams.
Results
Simulated weekly neutron ambient dose equivalents at 30 cm outside vault walls and the door ranged from background levels up to 0.065 mSv wk⁻¹. These results indicate that standard LINAC vault shielding is sufficient for most perimeter locations; however, additional shielding is required for a wall adjacent to a nearby LINAC room to satisfy the 0.02 mSv wk⁻¹ public dose limit. Additional radiation survey in patient QA modes of coplanar and non-coplanar sacrum planned beams implied neutron dose rates below 100 mrem h⁻¹ at 50 cm from a compact patient-center model during beam-on conditions. The same method also proved a consistency with values reported for commercial compact proton therapy systems.
Conclusion
Monte Carlo simulations validated by clinical measurements demonstrate that a compact gantry-less synchrotron-based proton therapy system can be safely retrofitted into a conventional LINAC vault with minimal additional shielding. These findings support the feasibility of gantry-less proton therapy installations in existing radiotherapy facilities, potentially lowering infrastructure barriers and expanding access to proton therapy.