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

Subjects: Nuclear Science and Technology >> Radiation Physics and Technology submitted time 2023-05-31

Luo, Ying Hunag, Shengcong Zhang, Hui Mao, Haijun Li, Qiang Dai, Zhongying

Abstract： 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.

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