Assessment of the induced radioactivity in the treatment room of the heavy-ion medical machine in Wuwei using PHITS

Carbon-ion radiotherapy (CIRT) offers unique physical and biological advantages over photon radiotherapy. However, some materials and devices in the CIRT treatment room become radioactive under bombardment by therapeutic carbon-ion beams due to nuclear reactions, thereby leading to possible radiation hazards to medical staff and additional and unwanted doses to patients. This study assessed the level of induced radioactivity in the treatment room of the Heavy-Ion Medical Machine (HIMM) in Wuwei. Monte Carlo simulations using PHITS were performed for a conservative case under the conditions of maximum beam energy and intensity provided by the HIMM facility. The geometry and configuration of Treatment Room 2 of the HIMM facility in Wuwei were adopted. We evaluated the activation of air, the phantom, and the components of the beamline, such as the primary collimator (PC), ridge filter (RF), and multileaf collimator (MLC). For air activation, we calculated the medical staff immersion external exposure and inhalation internal exposure caused by the corresponding radionuclides. For phantom activation, we estimated the additional dose to the patient’s family members owing to secondary photons after treatment. In addition, the exemption or non-exemption of the component material activation was assessed. The results showed that external radiation caused by air activation was the main source of the annual effective dose at approximately 0.5 mSv/y. The induced radioactivity exposure to family members of a patient after CIRT was approximately 40 μSv, sufficiently lower than the public dose limit of 1 mSv/a. The induced radioactivity of the PC, RF, and MLC was all above the exempt levels after the devices were retired, whereas the induced radioactivity of the RS and compensator could reach the exempt levels after one patient session. Our study indicated that medical staff engaged in CIRT should stay away from the high-dose-rate area of induced radioactivity along the beam direction, shorten the residence time in the treatment room as much as possible, and store the activated components in isolation after the equipment is out of use. Thus, this study provides guidance for accurately assessing the level of induced radioactivity in the treatment room for CIRT.