Investigation on the PBS FLASH Beam Delivery Technique Using a Proton Linac

Abstract

This study aims to investigate the use of proton Pencil Beam Scanning (PBS) for uniform FLASH irradiations in clinical and (radio)biology settings using a pulsed proton therapy Linac and to report on PBS FLASH beam delivery possibilities and limitations. A treatment planning system was used to perform treatment plan calculations to deliver 10 to 40 Gy uniform doses to various clinical and reference targets at shallow, medium and deep ranges. The design parameters from a proton Linac were considered to deliver the needed doses within the 0.5 s FLASH time limit at a pulse rate of 200 Hz. Ultra-high dose rate (UHDR) requires a high number of protons per pulse. We consider up to 1600 Mp (mega proton) per pulse, assuming 1 pulse per spot. The analysis was performed for a standard cyclotron beam model of 4 mm σ and proton Linac beam models of 1, 3 and 6 mm σ. Our analytical study demonstrates that proton Linac could deliver ultra-high doses within 0.5 s with dose rates of the order of 90 Gy/s and the same time structure of 3 GHz conventional electron Linac. Optimized parameters such as spot spacing and size can contribute to shorter delivery time needed to produce the FLASH effect with protons using a clinical machine setting. As FLASH transitions from pre-clinical to clinical use, existing systems may have potential limitations in clinical FLASH delivery. These include unwanted entrance dose and restricted UHDR at shallow-mid depths. The proton therapy Linac design promises high clinical FLASH performance with improved conformality and UHDR invariance with depth. The flexibility to selectively change the beam size to 6 mm σ suggests reduced PBS delivery time allowing FLASH irradiation of larger targets. The proton Linac and its successive evolutions are subject to conformity assessment and market authorization.