Casares-Magaz Oscar, Toftegaard Jakob, Muren Ludvig P, Kallehauge Jesper F, Bassler Niels, Poulsen Per R, Petersen Jørgen B B
Department of Medical Physics, Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
Acta Oncol. 2014 Aug;53(8):1058-63. doi: 10.3109/0284186X.2014.927586. Epub 2014 Jun 30.
Proton therapy offers the potential for sparing the normal tissue surrounding the target. However, due to well-defined proton ranges around the Bragg peak, dose deposition is more sensitive to changes in the water equivalent path length (WEPL) than with photons. In this study, we assess WEPL variations caused by breathing-induced motion for all possible beam angles in a series of lung cancer patients. By studying the association between measures for WEPL variation and breathing-induced target dose degradation we aimed to develop and explore a tool to identify beam angles that are robust to patient-specific patterns of intra-fractional motion.
Using four-dimensional computed tomography (4DCT) images of three lung cancer patients we evaluated the impact of the WEPL changes on target dose coverage for a series of coplanar single-beam plans. The plans were optimised for the internal target volume (ITV) at the maximum intensity projection (MIP) CT for every 3° gantry interval. The plans were transferred to the ten 4DCT phases and the average reduction in ITV V₉₅ over the ten phases, relative to the original MIP CT calculation, was quantified. The target dose reduction was associated with the mean difference between the WEPL and the phase-averaged WEPL computed for all beam rays across all possible gantry-couch angle combinations.
The gantry-couch angle maps showed areas of both high and low WEPL variation, with overall quite similar patterns yet with individual differences reflecting differences in tumour position and breathing-induced motion. The coplanar plans showed a strong association between WEPL changes and ITV V₉₅ reduction, with a correlation coefficient ranging between 0.92 and 0.98 for the three patients (p < 0.01).
We have presented a 4DCT-based method to quantify WEPL changes during the breathing cycle. The method identified proton field gantry-couch angle combinations that were either sensitive or robust to WEPL changes. WEPL variations along the beam path were associated with target under-dosage.
质子治疗具有保护靶区周围正常组织的潜力。然而，由于布拉格峰周围质子射程明确，与光子相比，剂量沉积对水等效路径长度（WEPL）的变化更为敏感。在本研究中，我们评估了一系列肺癌患者在所有可能射束角度下呼吸诱导运动引起的WEPL变化。通过研究WEPL变化测量值与呼吸诱导的靶区剂量降解之间的关联，我们旨在开发并探索一种工具，以识别对患者特定的分次内运动模式具有鲁棒性的射束角度。
使用三名肺癌患者的四维计算机断层扫描（4DCT）图像，我们评估了一系列共面单射束计划中WEPL变化对靶区剂量覆盖的影响。这些计划针对每个3°机架间隔的最大强度投影（MIP）CT上的内部靶区体积（ITV）进行了优化。将这些计划转移到十个4DCT相位，并相对于原始MIP CT计算，量化十个相位上ITV V₉₅的平均降低。靶区剂量降低与针对所有可能的机架-治疗床角度组合计算的所有射束射线的WEPL与相位平均WEPL之间的平均差异相关。
机架-治疗床角度图显示了WEPL变化的高值和低值区域，总体模式相当相似，但个体差异反映了肿瘤位置和呼吸诱导运动的差异。共面计划显示WEPL变化与ITV V₉₅降低之间存在很强的关联，三名患者的相关系数在0.92至0.98之间（p < 0.01）。
我们提出了一种基于4DCT的方法来量化呼吸周期中的WEPL变化。该方法识别出了对WEPL变化敏感或鲁棒的质子野机架-治疗床角度组合。沿射束路径的WEPL变化与靶区剂量不足相关。
