背景:用于FLASH放射治疗(RT)的超高剂量率(UHDR)电子加速器通过电子束与加速器内部和外部物体的相互作用而产生非常强烈的致辐射。每个脉冲的致辐射剂量通常比相同能量的常规RTX射线治疗的剂量大1-2个数量级,对于10MeV以上的电子能量,与常规RT相比,the致辐射在加速器外部产生的诱导放射性要多得多。因此,在UHDR电子加速器运行之前,必须进行全面的辐射安全评估。
目的:评估具有FLASH功能的VarianTrueBeam加速器原型的辐射安全性,并开发一个通用框架,用于评估FLASHRT的UHDR电子加速器的所有关键辐射安全性。
方法:通过各种分析方法对通过UHDR电子加速器产生的致辐射和诱导的放射性进行建模。将分析模型与国家标准与技术研究所(NIST)的致辐射产量数据以及掩体外部的一次致辐射测量值以及具有FLASH功能的16MeVVarianTrueBeam电子加速器的辐照厚目标的诱导放射性进行比较。此外,通过对掩体内部/外部的次生致辐射和迷宫入口处的中子的测量来补充分析建模。
结果:计算的致辐射产量与NIST数据的最大偏差为8.5%,所有的测量结果和诱导的放射性与计算一致,验证分析工具。此外,发现散射箔致辐射主要是一次致辐射,二次致辐射的主要来源是加速器外部的被辐照物体。因此,可以使用与常规RT相同的简单形式主义来计算掩体外部的主要和次要致辐射。简单形式主义的混凝土的测得的主致辐射十值层与NCRP和IAEA数据非常吻合,而测得的混凝土次致辐射第十值层远低于NCRP和IAEA的数据。所有计算和测量都形成了评估UHDR电子加速器所有关键辐射安全特性的总体框架。
结论:启用FLASH的VarianTrueBeam加速器对于正常操作是安全的(最大。每次照射99个脉冲)在为至少15MV常规X射线治疗而设计的掩体中,除非UHDR工作量比X射线工作量大得多。类似的发现适用于其他UHDR电子加速器。然而,在光束调谐期间,辐射调查,或其他延长辐照时间的测试,UHDR工作负载可能变得非常大,需要实施额外的安全措施。
BACKGROUND: An ultra-high dose rate (UHDR) electron accelerator for FLASH radiotherapy (RT) produces very intense bremsstrahlung by the interaction of the electron beam with objects both inside and outside of the accelerator. The bremsstrahlung dose per pulse is typically 1-2 orders of magnitude larger than that of conventional RT x-ray treatment of the same energy, and for electron energies above 10 MeV, the bremsstrahlung produces substantially more induced radioactivity outside the accelerator than for conventional RT. Therefore, a thorough radiation safety assessment is mandatory prior to the operation of a UHDR electron accelerator.
OBJECTIVE: To evaluate the radiation safety of a prototype FLASH-enabled Varian TrueBeam accelerator and to develop a general framework for assessment of all key radiation safety properties of a UHDR electron accelerator for FLASH RT.
METHODS: Production of bremsstrahlung and induced radioactivity by a UHDR electron accelerator is modeled by various analytical methods. The analytical modeling is compared with National Institute of Standards and Technology (NIST) bremsstrahlung yield data as well as measurements of primary bremsstrahlung outside the bunker and induced radioactivity of irradiated thick targets for a FLASH-enabled 16 MeV Varian TrueBeam electron accelerator. In addition, the analytical modeling is complemented by measurements of secondary bremsstrahlung inside/outside the bunker and neutrons at the maze entrance.
RESULTS: Calculated bremsstrahlung yields deviate maximum 8.5% from NIST data, and all measurements of primary bremsstrahlung and induced radioactivity agree with calculations, validating the analytical tools. In addition, it is found that scattering foil bremsstrahlung dominates primary bremsstrahlung and the main source of secondary bremsstrahlung is the irradiated object outside the accelerator. It follows that primary and secondary bremsstrahlung outside the bunker can be calculated using the same simple formalism as that used for conventional RT. Measured primary bremsstrahlung tenth-value layers for concrete of the simple formalism are in good agreement with NCRP and IAEA data, while measured secondary bremsstrahlung tenth-value layers for concrete are considerably lower than NCRP and IAEA data. All calculations and measurements form a general framework for assessment of all key radiation safety properties of a UHDR electron accelerator.
CONCLUSIONS: The FLASH-enabled Varian TrueBeam accelerator is safe for normal operation (max. 99 pulses per irradiation) in a bunker designed for at least 15 MV conventional x-ray treatment unless the UHDR workload is much larger than the x-ray workload. A similar finding applies to other UHDR electron accelerators. However, during beam tuning, radiation survey, or other tests with extended irradiation time, the UHDR workload may become very large, necessitating the implementation of additional safety measures.