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内容記述 |
At the National Institutes for Quantum Science and Technology (QST), a multi-ion therapy using helium, carbon, oxygen, and neon ions has been studied for charged particle therapy with more optimal biological effects. To make multi-ion therapy clinically feasible, a new treatment system was developed to realize the changes of the ion species in each irradiation using the Heavy Ion Medical Accelerator in Chiba (HIMAC). Since radiation therapy is safety-critical, it is necessary to construct a safety system that includes multiple safety barriers in the new treatment system for multi-ion therapy and to perform a safety analysis for the prevention of serious accidents. In this study, we conducted a safety analysis using Event Tree Analysis (ETA) for newly introduced processes in the treatment planning, accelerator, and irradiation system of the multi-ion therapy. ETA is the optimal method to verify the multiple safety barriers that are essential for medical safety, and the time for safety analysis can be shortened by focusing only on new processes. By using ETA, we clarified what kinds of malfunctions and human errors may lead to serious accidents in the new system for multi-ion therapy, and we verified whether the safety barriers such as interlock systems and human check procedures are sufficient to prevent such malfunctions and human errors. As a result, 6 initial events which may lead to serious accidents were listed in the treatment planning process, 16 initial events were listed in the accelerator system, and 13 initial events were listed in the irradiation system. Among these 35 initial events of the whole process, 5 cautionary initial events were identified that could lead to serious final events and they had a probability of occurrence higher than 10-4. The other 30 were all initial events that do not lead to serious accidents, or the initial events that can lead to serious accidents but were considered to have sufficient safety barriers. The safety analysis using ETA successfully determined the system malfunctions and the human errors that can lead to serious accidents, and the multiple safety barriers against them were systematically analyzed. It became clear that the multiple safety barriers were not sufficient for some initial events. We plan to improve the safety barriers for the five cautionary initial events before the start of the clinical trial. Based on these findings, we achieved our objective to conduct a safety analysis for a new treatment system for multi-ion therapy. The safety analysis procedure using ETA proposed by this study will be effective when new systems for radiotherapy are established at QST and other facilities in the future as well. |
