Bäumer Christian, Plaude Sandija, Khalil Dalia Ahmad, Geismar Dirk, Kramer Paul-Heinz, Kröninger Kevin, Nitsch Christian, Wulff Jörg, Timmermann Beate
West German Proton Therapy Centre Essen, Essen, Germany.
West German Cancer Center (WTZ), University Hospital Essen, Essen, Germany.
Front Oncol. 2021 May 12;11:599018. doi: 10.3389/fonc.2021.599018. eCollection 2021.
Proton therapy makes use of the favorable depth-dose distribution with its characteristic Bragg peak to spare normal tissue distal of the target volume. A steep dose gradient would be desired in lateral dimensions, too. The widespread spot scanning delivery technique is based, however, on pencil-beams with in-air spot full-widths-at-half-maximum of typically 1 cm or more. This hampers the sparing of organs-at-risk if small-scale structures adjacent to the target volume are concerned. The trimming of spot scanning fields with collimating apertures constitutes a simple measure to increase the transversal dose gradient. The current study describes the clinical implementation of brass apertures in conjunction with the pencil-beam scanning delivery mode at a horizontal, clinical treatment head based on commercial hardware and software components. Furthermore, clinical cases, which comprised craniopharyngiomas, re-irradiations and ocular tumors, were evaluated. The dosimetric benefits of 31 treatment plans using apertures were compared to the corresponding plans without aperture. Furthermore, an overview of the radiation protection aspects is given. Regarding the results, robust optimization considering range and setup uncertainties was combined with apertures. The treatment plan optimizations followed a single-field uniform dose or a restricted multi-field optimization approach. Robustness evaluation was expanded to account for possible deviations of the center of the pencil-beam delivery and the mechanical center of the aperture holder. Supplementary apertures improved the conformity index on average by 15.3%. The volume of the dose gradient surrounding the PTV (evaluated between 80 and 20% dose levels) was decreased on average by 17.6%. The mean dose of the hippocampi could be reduced on average by 2.9 GyRBE. In particular cases the apertures facilitated a sparing of an organ-at-risk, e.g. the eye lens or the brainstem. For six craniopharyngioma cases the inclusion of apertures led to a reduction of the mean dose of 1.5 GyRBE (13%) for the brain and 3.1 GyRBE (16%) for the hippocampi.
质子治疗利用其具有特征性布拉格峰的良好深度剂量分布，以保护靶区远端的正常组织。在横向维度上也需要陡峭的剂量梯度。然而，广泛应用的点扫描递送技术基于笔形束，其在空气中的点半高宽通常为1厘米或更大。如果涉及到与靶区相邻的小尺度结构，这会妨碍对危及器官的保护。用准直孔径修整点扫描野是增加横向剂量梯度的一种简单措施。本研究描述了基于商业硬件和软件组件，在水平临床治疗头中，将黄铜孔径与笔形束扫描递送模式相结合的临床应用。此外，还对包括颅咽管瘤、再照射和眼部肿瘤在内的临床病例进行了评估。将31个使用孔径的治疗计划的剂量学益处与相应的无孔径计划进行了比较。此外，还给出了辐射防护方面的概述。关于结果，将考虑射程和设置不确定性的稳健优化与孔径相结合。治疗计划优化遵循单野均匀剂量或受限多野优化方法。稳健性评估扩展到考虑笔形束递送中心与孔径支架机械中心的可能偏差。辅助孔径平均使适形指数提高了15.3%。PTV周围剂量梯度的体积（在80%和20%剂量水平之间评估）平均减少了17.6%。海马体的平均剂量平均可降低2.9 GyRBE。在特定情况下，孔径有助于保护危及器官，例如晶状体或脑干。对于6例颅咽管瘤病例，使用孔径导致大脑的平均剂量降低1.5 GyRBE（13%），海马体的平均剂量降低3.1 GyRBE（16%）。
