Bragg peak FLASH radiotherapy (RT) uses a distal tracking method to eliminate exit doses and can achieve superior OAR sparing. This study explores the application of this novel method in stereotactic body radiotherapy prostate FLASH-RT. An in-house platform was developed to enable intensity-modulated proton therapy (IMPT) planning using a single-energy Bragg peak distal tracking method. The patients involved in the study were previously treated with proton stereotactic body radiotherapy (SBRT) using the pencil beam scanning (PBS) technique to 40 Gy in five fractions. FLASH plans were optimized using a four-beam arrangement to generate a dose distribution similar to the conventional opposing beams. All of the beams had a small angle of two degrees from the lateral direction to increase the dosimetry quality. Dose metrics were compared between the conventional PBS and the Bragg peak FLASH plans. The dose rate histogram (DRVH) and FLASH metrics of 40 Gy/s coverage (V40Gy/s) were investigated for the Bragg peak plans. There was no significant difference between the clinical and Bragg peak plans in rectum, bladder, femur heads, large bowel, and penile bulb dose metrics, except for Dmax. For the CTV, the FLASH plans resulted in a higher Dmaxthan the clinical plans (116.9% vs. 103.3%). For the rectum, the V40Gy/sreached 94% and 93% for 1 Gy dose thresholds in composite and single-field evaluations, respectively. Additionally, the FLASH ratio reached close to 100% after the application of the 5 Gy threshold in composite dose rate assessment. In conclusion, the Bragg peak distal tracking method can yield comparable plan quality in most OARs while preserving sufficient FLASH dose rate coverage, demonstrating that the ultra-high dose technique can be applied in prostate FLASH SBRT.
布拉格峰FLASH放射治疗采用远端追踪方法消除出射剂量，可实现更优的危及器官保护效果。本研究探索该创新方法在立体定向体部放射治疗前列腺FLASH-RT中的应用。通过自主研发平台，实现了基于单能量布拉格峰远端追踪方法的调强质子治疗计划设计。研究纳入既往接受铅笔束扫描质子立体定向放射治疗的患者（5次分割，总剂量40 Gy）。FLASH计划采用四野照射布局进行优化，以生成与传统对穿野相似的剂量分布。所有射束均从侧向偏移2度小角度以提升剂量学质量。通过对比传统铅笔束扫描计划与布拉格峰FLASH计划的剂量学参数，并分析布拉格峰计划的剂量率直方图及40 Gy/s覆盖率等FLASH指标，发现除Dmax外，两组计划在直肠、膀胱、股骨头、大肠及阴茎球的剂量参数均无显著差异。对于临床靶区，FLASH计划的Dmax高于传统计划（116.9%对比103.3%）。在剂量率分析中，直肠的V40Gy/s在复合评估与单野评估中分别达到94%和93%（1 Gy剂量阈值）。此外，采用5 Gy阈值进行复合剂量率评估时，FLASH比率接近100%。研究表明，布拉格峰远端追踪方法能在保持充分FLASH剂量率覆盖的同时，使多数危及器官获得与传统计划相当的剂量学质量，证实该超高剂量率技术可应用于前列腺FLASH立体定向放射治疗。
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