Centre for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland. Physics Department, ETH Zürich, Zürich, Switzerland.
Phys Med Biol. 2018 Dec 20;64(1):015002. doi: 10.1088/1361-6560/aaf2e8.
In proton therapy, the lateral fall-off is often used to spare critical organs. It is therefore crucial to improve the penumbra for proton pencil beam scanning. However, previous work has shown that collimation may not be necessary for depths of >15 cm in water. As such, in this work we investigate the effectiveness of a thin multi leaf collimator (just thick enough to completely stop protons with ranges of <15 cm in water) for energy layer specific collimation in patient geometries, when applied in combination with both grid and contour scanned PBS proton therapy. For this, an analytical model of collimated beam shapes, based solely on data available in the treatment planning system, has been included in the optimization, with the resulting optimised plans then being recalculated using Monte Carlo in order to most accurately simulate the full physics effects of the collimator. For grid based scanning, energy specific collimation has been found to reduce the V30 outside the PTV by 19.8% for an example patient when compared to the same pencil beam placement without collimation. V30 could be even reduced by a further 5.6% when combining collimation and contour scanning. In addition, mixed plans, consisting of contour scanning for deep fields (max range >15 cm WER) and collimated contour scanning for superficial fields (<15 cm), have been created for four patients, by which V30 could be reduced by 0.8% to 8.0% and the mean dose to the brain stem by 1.5% to 3.3%. Target dose homogeneity however is not substantially different when compared to the best un-collimated scenario. In conclusion, we demonstrate the potential advantages of a thin, multi leaf collimator in combination with contour scanning for energy layer specific collimation in PBS proton therapy.
在质子治疗中，通常使用侧向下降来保护关键器官。因此，改善质子笔形束扫描的半影是至关重要的。然而，以前的工作表明，在水的深度大于 15 厘米时，准直可能不是必需的。因此，在这项工作中，我们研究了在患者几何形状中，对于能量层特异性准直，在与网格和轮廓扫描 PBS 质子治疗结合使用时，薄多叶准直器（仅足够厚以完全阻止水的范围内小于 15 厘米的质子）的有效性。为此，在优化中包含了一个基于可用在治疗计划系统中的数据的准直束形状的分析模型，并用蒙特卡罗方法重新计算了优化后的计划，以最准确地模拟准直器的全物理效应。对于基于网格的扫描，与没有准直的相同铅笔束放置相比，对于一个示例患者，能量特异性准直已被发现将 PTV 外部的 V30 降低了 19.8%。当结合准直和轮廓扫描时，V30 甚至可以进一步降低 5.6%。此外，通过为四个患者创建混合计划（深部场（最大范围> 15 厘米 WER）的轮廓扫描和浅层场（<15 厘米）的准直轮廓扫描），V30 可以降低 0.8%至 8.0%，脑干的平均剂量可以降低 1.5%至 3.3%。然而，与最佳未准直情况相比，目标剂量均匀性并没有显著不同。总之，我们证明了在 PBS 质子治疗中，薄的多叶准直器与轮廓扫描相结合用于能量层特异性准直的潜在优势。
