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

Objective Addressing the issue of induced radioactivity in carbon ion therapy system operation, to systematically evaluate the impact of beam parameters on the distribution of induced radiation field in carbon ion therapy room 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 and irradiation durations. Results The induced radioactivity generated by components exhibited a linear response to beam intensity. The MLC surface exhibited a residual dose rate of 15.8 μSv/h at 1 min post-irradiation (1×10 ⁸ pps, 15-minute irradiation), requiring 45 min cooling for the safety threshold. The activation nuclides produced in patient phantoms were dominated by short-lived nuclides, 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 patient phantoms. Personnel should avoid direct contact within 10 minutes post-irradiation. Occupational exposure levels of the air activation remain compliant with occupational dose constraints.