Medical Physics Division, Institute for Cancer Research and Treatment (IRCCS), 10060 Candiolo (TO), Italy.
Med Phys. 2013 Dec;40(12):121711. doi: 10.1118/1.4829515.
Pretreatment patient plan verification with gamma index (GI) metric analysis is standard procedure for intensity modulated radiation therapy (IMRT) treatment. The aim of this paper is to evaluate the variability of the local and global gamma index obtained during standard pretreatment quality assurance (QA) measurements for plans performed with Tomotherapy unit. The QA measurements were performed with a 3D diode array, using variable passing criteria: 3%∕3 mm, 2%∕2 mm, 1%∕1 mm, each with both local and global normalization.
The authors analyzed the pretreatment QA results for 73 verifications; 37 were prostate cancer plans, 16 were head and neck plans, and 20 were other clinical sites. All plans were treated using the Tomotherapy Hi-Art System. Pretreatment QA plans were performed with the commercially available 3D diode array ArcCHECK™. This device has 1386 diodes arranged in a helical geometry spaced 1 cm apart. The dose measurements were acquired on the ArcCHECK™ and then compared with the calculated dose using the standard gamma analysis method. The gamma passing rate (%GP), defined as the percentage of points satisfying the condition GI < 1, was calculated for different criteria (3%∕3 mm, 2%∕2 mm, 1%∕1 mm) and for both global and local normalization. In the case of local normalization method, the authors set three dose difference threshold (DDT) values of 2, 3, and 5 cGy. Dose difference threshold is defined as the minimum absolute dose error considered in the analysis when using local normalization. Low-dose thresholds (TH) of 5% and 10% were also applied and analyzed.
Performing a paired-t-test, the authors determined that the gamma passing rate is independent of the threshold values for all of the adopted criteria (5%TH vs 10%TH, p > 0.1). Our findings showed that mean %GPs for local (or global) normalization for the entire study group were 93% (98%), 84% (92%), and 66% (61%) for 3%∕3 mm, 2%∕2 mm, and 1%∕1 mm criteria, respectively. DDT was equal to 2 cGy for the local normalization analysis cases. The authors observed great variability in the resulting %GP. With 3%∕3 mm gamma criteria, the overall passing rate with local normalization was 4.6% less on the average than with global one, as expected. The wide difference between %GP calculated with global or local approach is also confirmed by an unpaired t-test statistical analysis.
The variability of %GP obtained confirmed the necessity to establish defined agreement criteria that could be universal and comparable between institutions. In particular, while the gamma passing rate does not depend on the choice of threshold, the choice of DDT strongly influences the gamma passing rate for local calculations. The difference between global and local %GP was statistically significant for prostate and other treatment sites when DDT was changed from 2 to 3 cGy.
使用伽马指数（GI）度量分析对预处理患者计划进行验证是调强放射治疗（IMRT）治疗的标准程序。本文的目的是评估在 Tomotherapy 单元上执行的计划进行标准预处理质量保证（QA）测量期间获得的局部和全局伽马指数的可变性。QA 测量使用 3D 二极管阵列进行，使用可变通过标准：3%/3mm、2%/2mm、1%/1mm，每个标准均采用局部和全局归一化。
作者分析了 73 次验证的预处理 QA 结果；37 例为前列腺癌计划，16 例为头颈部计划，20 例为其他临床部位。所有计划均使用 Tomotherapy Hi-Art 系统进行治疗。使用市售的 3D 二极管阵列 ArcCHECK™进行预处理 QA 计划。该设备具有以螺旋几何形状布置的 1386 个二极管，间隔 1cm。在 ArcCHECK™上进行剂量测量，然后使用标准伽马分析方法与计算剂量进行比较。不同标准（3%/3mm、2%/2mm、1%/1mm）和局部和全局归一化的伽马通过率（%GP）定义为满足条件 GI<1 的点的百分比。在局部归一化方法的情况下，作者设置了三个剂量差阈值（DDT）值 2、3 和 5cGy。剂量差阈值定义为使用局部归一化时分析中考虑的最小绝对剂量误差。还应用并分析了低剂量阈值（TH）为 5%和 10%。
通过配对 t 检验，作者确定在所有采用的标准下（5%TH 与 10%TH，p>0.1），伽马通过率与阈值值无关。我们的研究结果表明，对于整个研究组，局部（或全局）归一化的平均%GP 分别为 3%/3mm、2%/2mm 和 1%/1mm 标准的 93%（98%）、84%（92%）和 66%（61%）。对于局部归一化分析情况，DDT 等于 2cGy。作者观察到结果%GP 的很大变化。使用 3%/3mm 伽马标准，局部归一化的总体通过率平均比全局归一化低 4.6%，这是预期的。通过未配对 t 检验统计分析也证实了全局和局部方法计算的%GP 之间的差异很大。
所获得的%GP 的可变性证实了需要建立可以在机构之间通用和可比的定义性协议标准的必要性。特别是，虽然伽马通过率不取决于阈值的选择，但 DDT 的选择强烈影响局部计算的伽马通过率。当 DDT 从 2 变为 3cGy 时，前列腺和其他治疗部位的全局和局部%GP 之间存在统计学显著差异。
Zotero 插件