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Abstract：
OBJECTIVE: Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH-RT dose rates (~200× clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH-RT dose rates.
METHODS: We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured.
RESULTS: When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH-RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1-h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv.
CONCLUSIONS: Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.
METHODS: We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured.
RESULTS: When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH-RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1-h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv.
CONCLUSIONS: Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.
摘要：
目标：由于它们的范围有限，在计算兆伏能量直线加速器拱顶中的屏蔽要求时，通常会忽略电子。然而,在FLASH-RT剂量率（〜200×临床剂量率）下操作时，不需要考虑16MeV电子的假设不成立，其中，致辐射光子的剂量率比设计屏蔽的18MV光束的剂量率高一个数量级。我们研究了将Varian21EX直线加速器转换为FLASH-RT剂量率的屏蔽和辐射防护影响。
方法：我们使用FlukeBiomedicalInovision451P测量仪和广能中子探测仪（Wendi）-2FHT762中子探测器在所有占用区域进行了辐射测量。还测量了1.8分钟FLASH-RT递送后激活的直线加速器成分的剂量率。
结果：当在180°的机架角度下操作时，例如在生物学实验期间，16MeVFLASH-RT电子在受控区域输送~10µSv/h，在不受控区域输送780µSv/h，在任何1小时USNRC限制下都高于20µSv。然而,超过20µSv,该装置必须连续运行92秒，在这个掩体和FLASH-RT波束中对应于等中心的3180Gy工作负载，由于实验性的物流，在该时间范围内交付是不可行的。虽然波束转向和剂量测定活动可能需要如此大的工作量，在这些活动中,机架位于0°处,并且不受控制区域中的剂量率变得不可检测。同样，在FLASH-RT交付后，直线加速器组件的中子激活可以在等中心附近达到25µSv/h，但在几分钟内消散，一小时内的总剂量低于20µSv。
结论：16MeVFLASH-RT电子束产生的致辐射光子在受控和不受控区域产生相应的剂量率，和保险库中激活的直线加速器组件。虽然我们的直线加速器保险库屏蔽被证明足够了，其他调查人员将谨慎确认其辐射安全计划的充分性，特别是如果在设计为6MV的保险库中操作。
方法：我们使用FlukeBiomedicalInovision451P测量仪和广能中子探测仪（Wendi）-2FHT762中子探测器在所有占用区域进行了辐射测量。还测量了1.8分钟FLASH-RT递送后激活的直线加速器成分的剂量率。
结果：当在180°的机架角度下操作时，例如在生物学实验期间，16MeVFLASH-RT电子在受控区域输送~10µSv/h，在不受控区域输送780µSv/h，在任何1小时USNRC限制下都高于20µSv。然而,超过20µSv,该装置必须连续运行92秒，在这个掩体和FLASH-RT波束中对应于等中心的3180Gy工作负载，由于实验性的物流，在该时间范围内交付是不可行的。虽然波束转向和剂量测定活动可能需要如此大的工作量，在这些活动中,机架位于0°处,并且不受控制区域中的剂量率变得不可检测。同样，在FLASH-RT交付后，直线加速器组件的中子激活可以在等中心附近达到25µSv/h，但在几分钟内消散，一小时内的总剂量低于20µSv。
结论：16MeVFLASH-RT电子束产生的致辐射光子在受控和不受控区域产生相应的剂量率，和保险库中激活的直线加速器组件。虽然我们的直线加速器保险库屏蔽被证明足够了，其他调查人员将谨慎确认其辐射安全计划的充分性，特别是如果在设计为6MV的保险库中操作。
