Background: This study aims to present the feasibility of developing a synchrotron-based proton ultra-high dose rate (UHDR) pencil beam scanning (PBS) system. Methods: The RF extraction power in the synchrotron system was increased to generate 142.4 MeV pulsed proton beams for UHDR irradiation at ~100 nA beam current. The charge per spill was measured using a Faraday cup. The spill length and microscopic time structure of each spill was measured with a 2D strip transmission ion chamber. The measured UHDR beam fluence was used to derive the spot dwell time for pencil beam scanning. Absolute dose distributions at various depths and spot spacings were measured using Gafchromic films in a solid-water phantom. Results: For proton UHDR beams at 142.4 MeV, the maximum charge per spill is 4.96 ± 0.10 nC with a maximum spill length of 50 ms. This translates to an average beam current of approximately 100 nA during each spill. Using a 2 × 2 spot delivery pattern, the delivered dose per spill at 5 cm and 13.5 cm depth is 36.3 Gy (726.3 Gy/s) and 56.2 Gy (1124.0 Gy/s), respectively. Conclusions: The synchrotron-based proton therapy system has the capability to deliver pulsed proton UHDR PBS beams. The maximum deliverable dose and field size per pulse are limited by the spill length and extraction charge.
背景：本研究旨在探讨开发基于同步加速器的质子超高速率（UHDR）笔形束扫描（PBS）系统的可行性。方法：通过提升同步加速器系统中的射频引出功率，产生能量为142.4 MeV的脉冲质子束，在约100 nA束流强度下进行UHDR辐照。利用法拉第杯测量每次引出束团的电荷量，通过二维条带透射电离室测量束团长度及其微观时间结构。基于测得的UHDR束流注量推导笔形束扫描的驻留时间。在固体水模体中使用Gafchromic胶片测量不同深度与点间距下的绝对剂量分布。结果：对于能量为142.4 MeV的质子UHDR束流，单次引出最大电荷量为4.96 ± 0.10 nC，最大引出时长为50 ms，相当于每次引出期间平均束流强度约为100 nA。采用2×2点阵照射模式时，在5 cm和13.5 cm深度处单次引出剂量分别为36.3 Gy（726.3 Gy/s）和56.2 Gy（1124.0 Gy/s）。结论：基于同步加速器的质子治疗系统能够实现脉冲质子UHDR笔形束扫描照射。单脉冲可施照的最大剂量与照射野尺寸受限于引出时长与引出电荷量。
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