Bedford James L
Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London SM2 5PT, United Kingdom.
Phys Med Biol. 2025 Feb 6;70(4). doi: 10.1088/1361-6560/adaf04.
The exact temporal characteristics of beam delivery affect the efficacy and outcome of ultra-high dose rate (UHDR or 'FLASH') radiotherapy, mainly due to the influence of the beam pulse structure on mean dose rate. Single beams may also be delivered in separate treatment sessions to elevate mean dose rate. This paper therefore describes a model for pulse-by-pulse treatment planning and demonstrates its application by making some generic observations of the characteristics of FLASH radiotherapy with photons and protons.A beam delivery model was implemented into the AutoBeam (v6.3) inverse treatment planning system, so that the individual pulses of the delivery system could be explicitly described during optimisation. The delivery model was used to calculate distributions of time-averaged and dose-averaged mean dose rate and the dose modifying factor for FLASH was then determined and applied to dose calculated by a discrete ordinates Boltzmann solver. The method was applied to intensity-modulated radiation therapy with photons as well as to passive scattering and pencil beam scanning with protons for the case of a simple phantom geometry with a prescribed dose of 36 Gy in 3 fractions.Dose and dose rate are highest in the target region, so FLASH sparing is most pronounced around the planning target volume (PTV). When using a treatment session per beam, OAR sparing is possible more peripherally. The sparing with photons is higher than with protons because the dose to OAR is higher with photons.The framework provides an efficient method to determine the optimal technique for delivering clinical dose distributions using FLASH. The most sparing occurs close to the PTV for hypofractionated treatments.
束流输送的确切时间特性会影响超高剂量率（UHDR或“FLASH”）放疗的疗效和结果，主要是由于束流脉冲结构对平均剂量率的影响。单束流也可以在单独的治疗疗程中输送，以提高平均剂量率。因此，本文描述了一种逐脉冲治疗计划模型，并通过对光子和质子FLASH放疗的特性进行一些一般性观察来展示其应用。将束流输送模型应用于AutoBeam（v6.3）逆向治疗计划系统，以便在优化过程中能够明确描述输送系统的各个脉冲。该输送模型用于计算时间平均和剂量平均的平均剂量率分布，然后确定FLASH的剂量修正因子并应用于由离散坐标玻尔兹曼求解器计算的剂量。该方法应用于光子调强放射治疗以及质子的被动散射和笔形束扫描，针对的是简单体模几何形状，规定剂量为36 Gy，分3次照射。靶区内的剂量和剂量率最高，因此FLASH保护在计划靶体积（PTV）周围最为明显。当每束流使用一个治疗疗程时，在更外围区域有可能实现对危及器官（OAR）的保护。光子治疗的保护效果高于质子治疗，因为光子对OAR的剂量更高。该框架提供了一种有效的方法，用于确定使用FLASH输送临床剂量分布的最佳技术。在大分割治疗中，最明显的保护发生在靠近PTV的区域。
