Department of Nuclear Science, Faculty of Science, University of Colombo, Colombo 03, Sri Lanka.
Department of Nuclear Medicine, iThemba LABS, Faure, 7131, South Africa.
Biomed Phys Eng Express. 2021 Feb 12;7(2). doi: 10.1088/2057-1976/abe33d.
Prompt gamma detection during proton radiotherapy for range verification purposes will need to operate in both active and passive treatment beam environments. This paper describes prompt gamma measurements using a high resolution 2″ × 2″ LaBrdetector for a 200 MeV clinical passive-scatter proton beam. These measurements examine the most likely discrete prompt gamma rays emitted from tissue by detecting gammas produced in water, Perspex, carbon and liquid-nitrogen targets. Measurements were carried out at several positions around the depth corresponding to the location of the Bragg peak for water and Perspex targets in order to investigate prompt gamma emission as a function of depth along the beam path. This work also focused on validating the Geant4 Monte Carlo model of the passive-scatter proton beam line and LaBrdetector by making a direct comparison between the simulated and experimental results. The initial prompt gamma measurements were overwhelmed by the high amount of scattered radiation when measuring at isocenter, shifting the target further downstream from the final collimator significantly reduced the background radiation. Prompt gamma peaks were then clearly identified for the water, Perspex and graphite targets. The developed Geant4 Monte Carlo model was able to replicate the measured prompt gamma ray energy spectra, including production for important photopeaks to within 10%, except for the 4.44 MeV peak from the water target, which had more than a 50% overestimation of the number of produced prompt gamma rays. The prompt gamma measurements at various depths correlated well with the proton dose deposition; the 4.44 and 6.13 MeV photopeak profiles peaked within 1 cm of the Bragg peak and the Rvalue for the 3-7 MeV energy range predicted the proton range within 8 mm.
为了验证射程，在质子放射治疗中需要对质子束进行主动和被动治疗环境下的瞬发伽马探测。本文描述了使用高分辨率 2"×2"LaB 探测器对 200 MeV 临床被动散射质子束进行的瞬发伽马测量。这些测量通过探测水、有机玻璃、碳和液氮靶中产生的伽马射线，研究了组织中最有可能发射的离散瞬发伽马射线。在几个位置进行了测量，这些位置对应于水和有机玻璃靶的布拉格峰位置的深度，以研究沿束路径的深度作为函数的瞬发伽马发射。这项工作还侧重于通过对被动散射质子束线和 LaB 探测器的 Geant4 蒙特卡罗模型进行直接比较，验证该模型。当在等中心测量时，初始的瞬发伽马测量受到大量散射辐射的干扰，将靶体从最终准直器进一步向下游移动会显著降低背景辐射。然后，水、有机玻璃和石墨靶的瞬发伽马峰被清晰地识别出来。开发的 Geant4 蒙特卡罗模型能够复制测量的瞬发伽马射线能谱，包括对重要光峰的产生，误差在 10%以内，除了水靶的 4.44 MeV 光峰，其产生的瞬发伽马射线数量的高估超过 50%。在不同深度的瞬发伽马测量与质子剂量沉积相关良好；4.44 和 6.13 MeV 光峰轮廓在布拉格峰内 1 cm 处达到峰值，3-7 MeV 能区的 R 值预测质子射程的误差在 8mm 以内。
