Guangzhou Concord Cancer Center, Sino-Singapore Guangzhou Knowledge City, Guangzhou City, China.
J Appl Clin Med Phys. 2023 Sep;24(9):e14106. doi: 10.1002/acm2.14106. Epub 2023 Aug 18.
MatriXX ionization chamber array has been widely used for the composite dose verification of IMRT/VMAT plans. However, in addition to its dose response dependence on gantry angle, there seems to be an offset between the beam axis and measured dose profile by MatriXX for oblique beam incidence at various gantry angles, leading to unnecessary quality assurance (QA) fails. In this study, we investigated the offset at various setup conditions and how to eliminate or decrease it to improve the accuracy of MatriXX for IMRT/VMAT plan verification with original gantry angles.
We measured profiles for a narrow beam with MatriXX located at various depths in increments of 0.5 mm from the top to bottom of the sensitive volume of the array detectors and gantry angles from 0° to 360°. The optimal depth for QA measurement was determined at the depth where the measured profile had minimum offset.
The measured beam profile offset varies with incident gantry angle, increasing from vertical direction to lateral direction, and could be over 3 cm at vendor-recommended depth for near lateral direction beams. The offset also varies with depth, and the minimum offset (almost 0 for most oblique beams) was found to be at a depth of ∼2.5 mm below the vendor suggested depth, which was chosen as the optimal depth for all QA measurements. Using the optimal depth we determined, QA results (3%/2 mm Gamma analysis) were largely improved with an average of 99.4% gamma passing rate (no fails for 95% criteria) for 10 IMRT and VMAT plans with original gantry angles compared to 94.1% using the vendor recommended depth.
The improved accuracy and passing rate for QA measurement performed at the optimal depth with original gantry angles would lead to reduction in unnecessary repeated QA or plan changes due to QA system errors.
MatriXX 电离室阵列已广泛用于 IMRT/VMAT 计划的综合剂量验证。然而，除了其剂量响应对机架角度的依赖性之外，对于各个机架角度的斜入射束，MatriXX 似乎在束轴和测量剂量分布之间存在偏移，导致不必要的质量保证（QA）失败。在这项研究中，我们研究了在各种设置条件下的偏移量，以及如何消除或减小偏移量，以提高 MatriXX 对原始机架角度的 IMRT/VMAT 计划验证的准确性。
我们在阵列探测器的敏感体积的顶部到底部以 0.5mm 的增量测量了具有 MatriXX 的窄束的分布，并且机架角度从 0°到 360°。在 QA 测量的最佳深度是在测量的分布具有最小偏移的深度处确定的。
测量的束流分布偏移随入射机架角度而变化，从垂直方向到侧向方向增加，并且在侧向方向附近的供应商推荐深度处可超过 3cm。该偏移量还随深度而变化，并且在距供应商建议深度约 2.5mm 的深度处发现最小偏移量（对于大多数斜入射束几乎为 0），这被选为所有 QA 测量的最佳深度。使用我们确定的最佳深度，QA 结果（3%/2mm Gamma 分析）得到了很大改善，与使用供应商推荐的深度相比，10 个原始机架角度的 IMRT 和 VMAT 计划的平均 Gamma 通过率为 99.4%（95%标准下无失败）。
在原始机架角度下使用最佳深度进行 QA 测量的准确性和通过率的提高，将减少由于 QA 系统误差而导致的不必要的重复 QA 或计划更改。
