Seco Joao, Robertson Daniel, Trofimov Alexei, Paganetti Harald
Department of Radiation Oncology, Harvard Medical School and Massachusetts General Hospital, Boston, MA, USA.
Phys Med Biol. 2009 Jul 21;54(14):N283-94. doi: 10.1088/0031-9155/54/14/N01. Epub 2009 Jun 23.
Treatment delivery with active beam scanning in proton radiation therapy introduces the problem of interplay effects when pencil beam motion occurs on a similar time scale as intra-fractional tumor motion. In situations where fractionation may not provide enough repetition to blur the effects of interplay, repeated delivery or 'repainting' of each field several times within a fraction has been suggested. The purpose of this work was to investigate the effectiveness of different repainting strategies in proton beam scanning. To assess the dosimetric impact of interplay effects, we performed a series of simulations considering the following parameters: tumor motion amplitude, breathing period, asymmetry in the motion trajectory for the target and time required to change the beam energy for the delivery system. Several repainting strategies were compared in terms of potential vulnerability to a dose delivery error. Breathing motion perpendicular to the beam direction (representing superior-inferior type tumor motion in patients) was considered and modeled as an asymmetric sine function with a peak-to-peak amplitude of between 10 and 30 mm. The results show that motion effects cause a narrowing of the high-dose profile and widening of the penumbra. The 90% isodose area was reduced significantly when considering a large motion amplitude of 3 cm. The broadening of the penumbra appears to depend only on the amplitude of tumor motion (assuming harmonic motion). The delivered dose exhibits a shift of 10-15% of the tumor amplitude (or 1-5 mm) in the caudal direction due to breathing asymmetry observed for both sin(4)(x) and sin(6)(x) motion. Of the five repainting techniques studied, so-called 'breath sampling' turned out to be most effective in reducing dose errors with a minimal increase in treatment time. In this method, each energy level is repainted at several evenly spaced times within one breathing period. To keep dose delivery errors below 5% while minimizing treatment time, it is recommended that breath sampling repainting be employed using 5-10 paintings per field for an assumed tumor volume of 8.5 x 8.5 x 10 cm(3). For smaller tumor volumes more repaintings will be required, while for larger volumes five repaintings should be sufficient to achieve the required dose accuracy.
在质子放射治疗中，采用主动束扫描进行治疗时，当笔形束运动与分次内肿瘤运动发生在相似的时间尺度上时，就会出现相互作用效应的问题。在分次治疗可能无法提供足够的重复次数来模糊相互作用效应的情况下，有人建议在一个分次内对每个射野进行多次重复照射或“重绘”。这项工作的目的是研究质子束扫描中不同重绘策略的有效性。为了评估相互作用效应的剂量学影响，我们进行了一系列模拟，考虑了以下参数：肿瘤运动幅度、呼吸周期、靶区运动轨迹的不对称性以及治疗系统改变束能量所需的时间。比较了几种重绘策略在剂量传递误差潜在易感性方面的差异。考虑了垂直于束方向的呼吸运动（代表患者体内上下型肿瘤运动），并将其建模为峰峰值幅度在10至30毫米之间的不对称正弦函数。结果表明，运动效应会导致高剂量分布变窄和半影增宽。当考虑3厘米的大运动幅度时，90%等剂量面积显著减小。半影增宽似乎仅取决于肿瘤运动的幅度（假设为简谐运动）。由于观察到sin(4)(x)和sin(6)(x)运动的呼吸不对称性，所传递的剂量在尾侧方向上表现出肿瘤幅度的10 - 15%（或1 - 5毫米）的偏移。在所研究的五种重绘技术中，所谓的“呼吸采样”在减少剂量误差方面最为有效，且治疗时间增加最少。在这种方法中，每个能量水平在一个呼吸周期内的几个均匀间隔时间进行重绘。为了将剂量传递误差保持在5%以下并使治疗时间最小化，对于假定肿瘤体积为8.5×8.5×10 cm(3)的情况，建议每个射野采用5 - 10次重绘的呼吸采样重绘方法。对于较小的肿瘤体积，需要更多的重绘次数，而对于较大的体积，五次重绘应该足以达到所需的剂量精度。
