Institute of Medical Physics, School of Physics, The University of Sydney, Sydney, NSW, Australia.
School of Medicine, Taif University, Taif, Saudi Arabia.
J Appl Clin Med Phys. 2021 Jun;22(6):229-240. doi: 10.1002/acm2.13221. Epub 2021 May 5.
To investigate intrinsic sensitivity of an electronic portal imaging device (EPID) and the ArcCHECK detector and to use this in assessing their performance in detecting delivery variations for lung SBRT VMAT. The effect of detector spatial resolution and dose matrix interpolation on the gamma pass rate was also considered.
Fifteen patients' lung SBRT VMAT plans were used. Delivery variations (errors) were introduced by modifying collimator angles, multi-leaf collimator (MLC) field sizes and MLC field shifts by ±5, ±2, and ±1 degrees or mm (investigating 103 plans in total). EPID and ArcCHECK measured signals with introduced variations were compared to measured signals without variations (baseline), using OmniPro-I'mRT software and gamma criteria of 3%/3 mm, 2%/2 mm, 2%/1 mm, and 1%/1 mm, to test each system's basic performance. The measurement sampling resolution for each was also changed to 1 mm and results compared to those with the default detector system resolution.
Intrinsic detector sensitivity analysis, that is, comparing measurement to baseline measurement, rather than measurement to plan, demonstrated the intrinsic constraints of each detector and indicated the limiting performance that users might expect. Changes in the gamma pass rates for ArcCHECK, for a given introduced error, were affected only by dose difference (DD %) criteria. However, the EPID showed only slight changes when changing DD%, but greater effects when changing distance-to-agreement criteria. This is pertinent for lung SBRT where the minimum dose to the target will drop dramatically with geometric errors. Detector resolution and dose matrix interpolation have an impact on the gamma results for these SBRT plans and can lead to false positives or negatives in error detection if not understood.
The intrinsic sensitivity approach may help in the selection of more meaningful gamma criteria and the choice of optimal QA device for site-specific dose verification.
研究电子射野影像装置（EPID）和 ArcCHECK 探测器的固有灵敏度，并将其用于评估它们在检测肺部 SBRT VMAT 剂量分布中的性能。还考虑了探测器空间分辨率和剂量矩阵插值对伽马通过率的影响。
使用了 15 例肺部 SBRT VMAT 计划。通过改变准直器角度、多叶准直器（MLC）射野大小和 MLC 射野偏移±5、±2 和±1 度或毫米（共研究了 103 个计划）来引入剂量分布变化（误差）。使用 OmniPro-I'mRT 软件和 3%/3mm、2%/2mm、2%/1mm 和 1%/1mm 的伽马标准，将引入变化后的 EPID 和 ArcCHECK 测量信号与无变化（基线）的测量信号进行比较，以测试每个系统的基本性能。还将每个系统的测量采样分辨率更改为 1mm，并将结果与默认探测器系统分辨率的结果进行比较。
固有探测器灵敏度分析，即比较测量值与基线测量值，而不是与计划测量值，表明了每个探测器的固有限制，并指出了用户可能期望的限制性能。对于给定引入误差，ArcCHECK 的伽马通过率变化仅受剂量差（DD%）标准的影响。然而，EPID 仅在改变 DD%时显示出轻微的变化，但在改变距离一致性标准时则会产生更大的影响。这对于肺部 SBRT 非常重要，因为靶区的最小剂量会因几何误差而急剧下降。探测器分辨率和剂量矩阵插值对这些 SBRT 计划的伽马结果有影响，如果不了解这些影响，可能会导致错误检测中的假阳性或假阴性。
固有灵敏度方法可能有助于选择更有意义的伽马标准，并为特定部位剂量验证选择最佳的 QA 设备。
