Charlwood Frances C, Aitkenhead Adam H, Mackay Ranald I
Manchester Academic Health Science Centre (MAHSC), Faculty of Medical and Human Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom and Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester M20 4BX, United Kingdom.
Med Phys. 2016 Mar;43(3):1462-72. doi: 10.1118/1.4941957.
The lateral edge of a proton therapy beam is commonly used to achieve conformality to the treatment volume where critical structures reside close to the target. However, when treating shallow depths, the lateral edge of a pencil beam scanning (PBS) system may be broader than that of a double scattered (DS) system. Use of a range-shifter to degrade the beam and allow treatment of very shallow depths further blurs the lateral edge. The authors investigate the potential use of a collimator with a PBS system for delivery of 3D uniform dose-volumes to a water-tank phantom, identifying the key factors controlling the width of the lateral edge.
The geant4 application for tomographic emission (gate) Monte Carlo (MC) environment was used, following validation against previously published data. Key parameters for PBS beams were investigated to assess their impact on the lateral edge of both monoenergetic beams and uniform dose-volumes. These parameters included nozzle-to-surface distance (NSD), vacuum window-to-surface distance (VSD), use of a range-shifter, and spot optimization parameters.
The lateral edge of an uncollimated PBS beam is particularly sensitive to VSD and NSD. While use of a range-shifter blurs the lateral edge, collimation allows the edge to be sharpened to between 2 and 4 mm depending on the depth of the target. Optimization of the spot weightings alone can provide a penumbral width close to that of a single spot, but also leads to poorer uniformity near the edge of the target volume.
Collimation of PBS beams should be considered for superficial targets particularly for beams delivered through a range-shifter, since the resultant sharpening of the lateral edge will allow improved sparing of adjacent normal tissues. Further work is needed to develop collimators which are integrated into both nozzle designs and planning system optimization algorithms.
质子治疗束的外侧边缘通常用于实现与治疗体积的适形性，其中关键结构位于靶区附近。然而，在治疗浅深度时，笔形束扫描（PBS）系统的外侧边缘可能比双散射（DS）系统的更宽。使用射程移位器来降低束流并允许治疗非常浅的深度会进一步模糊外侧边缘。作者研究了在PBS系统中使用准直器向水箱体模输送三维均匀剂量体积的潜在用途，确定了控制外侧边缘宽度的关键因素。
在根据先前发表的数据进行验证之后，使用用于断层发射（GATE）蒙特卡罗（MC）环境的geant4应用程序。研究了PBS束的关键参数，以评估它们对单能束和均匀剂量体积的外侧边缘的影响。这些参数包括喷嘴到表面的距离（NSD）、真空窗到表面的距离（VSD）、射程移位器的使用以及光斑优化参数。
未准直的PBS束的外侧边缘对VSD和NSD特别敏感。虽然使用射程移位器会模糊外侧边缘，但准直可使边缘锐化至2至4毫米，具体取决于靶区的深度。仅优化光斑权重可提供接近单个光斑的半影宽度，但也会导致靶区体积边缘附近的均匀性变差。
对于浅表靶区，特别是对于通过射程移位器输送的束流，应考虑对PBS束进行准直，因为外侧边缘的锐化将有助于更好地保护相邻的正常组织。需要进一步开展工作来开发集成到喷嘴设计和治疗计划系统优化算法中的准直器。
