Department of Radiation Oncology, Erasmus MC Cancer Institute, Groene Hilledijk 301, 3075 EA, Rotterdam, The Netherlands.
Med Phys. 2017 Nov;44(11):5556-5562. doi: 10.1002/mp.12518. Epub 2017 Sep 18.
The EPID-based sub-arc verification of VMAT dose delivery requires synchronization of the acquired electronic portal images (EPIs) with the VMAT delivery, that is, establishment of the start- and stop-MU of the acquired images. To realize this, published synchronization methods propose the use of logging features of the linac or dedicated hardware solutions. In this study, we developed a novel, software-based synchronization method that only uses information inherently available in the acquired images.
The EPIs are continuously acquired during pretreatment VMAT delivery and converted into Portal Dose Images (PDIs). Sub-arcs of approximately 10 MU are then defined by combining groups of sequentially acquired PDIs. The start- and stop-MUs of measured sub-arcs are established in a synchronization procedure, using only dosimetric information in measured and predicted PDIs. Sub-arc verification of a VMAT dose delivery is based on comparison of measured sub-arc PDIs with synchronized, predicted sub-arc PDIs, using γ-analyses. To assess the accuracy of this new method, measured and predicted PDIs were compared for 20 clinically applied VMAT prostate cancer plans. The sensitivity of the method for detection of delivery errors was investigated using VMAT deliveries with intentionally inserted, small perturbations (25 error scenarios; leaf gap deviations ≤ 1.5 mm, leaf motion stops during ≤ 15 MU, linac output error ≤ 2%).
For the 20 plans, the average failed pixel rates (FPR) for full-arc and sub-arc dose QA were 0.36% ± 0.26% (1 SD) and 0.64% ± 0.88%, based on 2%/2 mm and 3%/3 mm γ-analyses, respectively. Small systematic perturbations of up to 1% output error and 1 mm leaf offset were detected using full-arc QA. Sub-arc QA was able to detect positioning errors in three leaves only during approximately 20 MU and small dose delivery errors during approximately 40 MU. In an ROC analysis, the area under the curve (AUC) for the combined full-arc/sub-arc approach was 0.90.
A novel method for sub-arc VMAT dose delivery verification with EPIDs is proposed, using only dosimetric information in acquired EPIs for synchronization. Especially in combination with full-arc QA, the established sensitivity for detection of very small errors is high, with also a high specificity.
基于 EPID 的 VMAT 剂量输送的子弧验证需要将所获取的电子射野影像（EPIs）与 VMAT 输送同步，即确定所获取的图像的起始 MU 和停止 MU。为了实现这一点，已发表的同步方法提出使用直线加速器的日志功能或专用硬件解决方案。在这项研究中，我们开发了一种新颖的基于软件的同步方法，该方法仅使用所获取的图像中固有的信息。
在 VMAT 预处理输送过程中连续获取 EPIs，并将其转换为电子射野影像剂量（PDI）。然后，通过组合顺序获取的 PDIs 来定义大约 10 MU 的子弧。在同步过程中，仅使用所测量和预测的 PDIs 中的剂量学信息来建立测量子弧的起始 MU 和停止 MU。基于γ分析，通过比较测量的子弧 PDI 与同步的、预测的子弧 PDI，对 VMAT 剂量输送进行子弧验证。为了评估这种新方法的准确性，比较了 20 个临床应用的 VMAT 前列腺癌计划的测量和预测的 PDI。使用故意插入的小扰动（25 个误差场景；叶间隙偏差≤1.5mm，叶运动停止时间≤15MU，直线加速器输出误差≤2%）进行 VMAT 输送，研究了该方法检测输送误差的灵敏度。
对于 20 个计划，基于 2%/2mm 和 3%/3mmγ分析，全弧和子弧剂量 QA 的平均失败像素率（FPR）分别为 0.36%±0.26%（1 SD）和 0.64%±0.88%。使用全弧 QA 可以检测到高达 1%的输出误差和 1mm 的叶片偏移的小系统性扰动。仅在大约 20MU 期间和大约 40MU 期间的小剂量输送误差期间，子弧 QA 能够检测到三个叶片中的定位误差。在 ROC 分析中，全弧/子弧组合方法的曲线下面积（AUC）为 0.90。
提出了一种使用仅在获取的 EPIs 中获取的剂量信息进行同步的新型 EPID 用于子弧 VMAT 剂量输送验证的方法。特别是与全弧 QA 结合使用时，检测非常小误差的灵敏度很高，特异性也很高。
