Study on Dose-rate Distribution and Induced Radioactivity in the Treatment Room of MRI-Guided Carbon-Ion Therapy

Abstract: Magnetic resonance imaging guided （MRI-guided）carbon-ion therapy, integrates the advantage of precise dose delivery and real-time monitoring, is becoming the most advanced radiotherapy modality. To address the radiation protection requirements of MRI-guided carbon-ion therapy system, in this work beam deflection, dose-rate distribution and induced radioactivity of MRI system in treatment room were systematically evaluate using Fluka Monta Carlo simulation. With magnetic field strength increasing, the carbon-ion beam exhibited a gradually increasing displacement along the -x direction, which consequently altered the dose-rate distribution in both the target region and the MRI pole-plate region. For a 400 MeV/u carbon ion beam, the maximum dose rate at water phantom (Eval 1) was approximately 15.8 to 16.3 μSv/h during the prompt phase. The two-dimensional dose-rate distribution in the treatment room shifted and the dose rate along the maze (Eval 4) direction increased with increased magnetic field strength. Nevertheless, the dose rates at the evaluated shielding points of interest, maze (Eval 4) and wall (Eval 5), were both far below 2.5 μSv/h, indicating a relatively limited radiation contribution toward the adjacent treatment room. Furthermore, the total induced radioactivity of the Holder was markedly higher than those of pole-plates (MRI1 and MRI2) for MRI system. When the main magnetic field strength increased from 0.35 T to 3.0 T, the total activities of radionuclides produced in MRI1 and MRI2, and the Holder decreased by approximately 7.9 %, 7.8 %, and 15.3 %, respectively. The long-term residual activity was dominated by ^55 Fe and ^54 Mn. Overall, with appropriate shielding design and adequate cooling-time control, the radiation risks associated with MRI-guided carbon-ion therapy system are expected to remain within an acceptable and manageable range. These results provide a useful reference for shielding optimization, equipment maintenance, and the formulation of decommissioning management strategies for MRI-guided carbon-ion therapy treatment room.