Simulation Analysis and Radiation Protection Study of Induced Radioactivity in Carbon Ion Therapy Systems

To systematically evaluate the impact of beam parameters on the spatiotemporal distribution of induced radioactivity in carbon ion therapy systems and develop radiation protection strategies. Methods: A geometric model integrating a multi-leaf collimator (MLC), compensator, patient phantom, and concrete shielding structure was developed using the FLUKA Monte Carlo simulation code. Systematic simulations were conducted to analyze residual dose rate decay kinetics and radionuclide activation profiles at critical components under 400 MeV/u carbon ion irradiation with beam intensities ranging from 1×10⁷ to 1×10⁸ particles per second (pps) and irradiation durations of 5 to15 minutes. Results: The induced radioactivity generated by components exhibits a linear response to beam intensity. The MLC surface exhibited a residual dose rate of 15.8 μSv/h at 1 min post-irradiation (1×10⁸ pps, 15-minute irradiation), requiring 45 min cooling for the safety threshold. Patient activation was dominated by short-lived radionuclides, with radiation dose decreased to acceptable level within 10 min post-irradiation. Occupational exposure assessments demonstrated annual effective doses of about 174 μSv (air immersion) and 0.31 μSv (inhalation). Conclusion: The induced radioactivity dose produced by the carbon ion radiotherapy system is mainly generated by MLC and patients. Personnel direct contact should be avoid right after the irradiation. Occupational exposure levels of the air activation remain compliant with occupational dose constraints.