Author: Chin-Cheng Chen, Chingyun Cheng, Longfei Diao, Benjamin Durkee, Minglei Kang, Haibo Lin, LeLe Liu, Yangguang Ma, Xuanqin Mou, YunTong Pei, Charles B. Simone, YuFei Wang, Xueqing Yan, Xingyi Zhao, Wang Zhengda 👨🔬
Affiliation: Peking University, St. Jude Children's Research Hospital, New York Proton Center, Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Department of Radiation Oncology, The First Affiliated Hospital of Zhengzhou University, School of Software Engineering, Xi’an Jiaotong University, School of information and communications engineering, Faculty of electronic and information engineering, Xi’an Jiaotong University, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Department of Human Oncology, University of Wisconsin-Madison 🌍
Purpose: This study evaluates the effectiveness of apertures in improving lateral dose falloff in proton pencil beam scanning (PBS) single-energy Bragg peak (SEBP) FLASH radiotherapy (RT). The approach aims to mitigate the increased lateral dose spillage associated with the use of universal range shifters (URS) and range compensators (RC).
Methods: PBS SEBP FLASH treatment plans were optimized both with and without brass apertures using an in-house treatment planning system. Dose and dose rate characteristics were simulated with the MCSquare Monte Carlo platform. Penumbra reduction was systematically analyzed in a water phantom for 3 cm and 5 cm square fields, under varying range pullback distances (10 cm, 20 cm, 30 cm) and air gap conditions (5 cm, 10 cm, 15 cm). The effects of apertures were evaluated at entrance, midpoint, and Bragg peak positions along the beam path. Additionally, an rGBM cancer treatment plan was optimized, exploring different dose thresholds (0 Gy, 2 Gy, and 5 Gy) to assess dose conformity and ultra-high dose rate performance (V40 Gy/s).
Results: Apertures reduced lateral penumbra at all positions, with greater reductions at larger range pullbacks. Penumbra reduction at Bragg peak was similar to entrance reduction. Larger field sizes (5 cm) demonstrated greater penumbra reduction compared to smaller field sizes (3 cm). For rGBM treatment, apertures enhanced gross tumor volume (GTV) dose conformity and organs at risk (OARs) sparing but reduced FLASH dose coverage in both GTV and OARs. The ultra-high dose rate coverage of the GTV remained unaffected by applying dose thresholds, whereas the ultra-high dose rate coverage of the brain increased with higher dose thresholds.
Conclusion: Apertures minimize lateral penumbra and dose spillage, improving target dose conformity in SEBP FLASH radiotherapy with range shifters. However, reduced ultra-high dose rate coverage requires careful evaluation to balance tumor control and the FLASH effect.