Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905, USA.
Med Phys. 2010 Jun;37(6):2516-24. doi: 10.1118/1.3425781.
To report on the sensitivity of single field planar measurements in identifying IMRT plans with poor calculational accuracy.
Three IMRT plans for head and neck cancer were subjected to extensive quality assurance. The plans were recalculated on a cylindrical phantom and between eight and 18 low gradient points were measured in each plan with an ion chamber. Every point measured in these plans agreed to within 4% of the dose predicted by the planning system and the plans were judged acceptable for clinical use. Each plan was then reoptimized with aggressive dose constraints so that the new treatment fields were more highly modulated than the ones from the original plans. Very complex fields can be calculated less accurately and ion chamber measurements of these plans in the cylindrical phantom confirmed significant dosimetric errors--Several of the measured points in each plan differed from the calculated dose by more than 4%, with a maximum single deviation of 10.6%. These three plans were judged unacceptable for clinical use. All six plans (three acceptable, three unacceptable) were then analyzed with two means of individual field planar dosimetry: Portal imaging with an electronic portal imaging device (EPID) and an ion chamber array. Gamma analysis was performed on each set of planar measurements with 2%/2 mm distance to agreement (DTA) and 3%/3 mm DTA criteria to try to determine a gamma analysis threshold which would differentiate the flawed plans from the acceptable ones.
With the EPID and 2%/2 mm DTA criteria, between 88.2% and 92.8% of pixels from the acceptable IMRT plans passed the gamma analysis, and between 87.5% and 91.9% passed for the unacceptable IMRT plans. With the ion chamber array and 2%/2 mm DTA criteria, between 92.4% and 94.9% of points in the acceptable plans passed the gamma analysis, while 86.8% to 98.3% of the points in the unacceptable plans passed the gamma analysis. The difference between acceptable and unacceptable plans was diminished further when gamma criteria were expanded to 3%/3 mm DTA. A fraction of pixels passing the gamma analysis was found to be a poor predictor of dosimetric accuracy with both planar dosimeters, as well as both sets of gamma criteria.
Deconstruction of an IMRT plan for field-by-field QA requires complex analysis methods such as the gamma function. Distance to agreement, a component of the gamma function, has clinical relevance in a composite plan but when applied to individual, highly modulated fields, it can mask important dosimetric errors. While single field planar dosimetry may comprise one facet of an effective QA protocol, gamma analysis of single field measurements is insensitive to important dosimetric inaccuracies of the overall plan.
报告单野平面测量在识别 IMRT 计划计算精度差中的灵敏度。
对头颈部癌症的三个 IMRT 计划进行了广泛的质量保证。计划在圆柱形体模上重新计算，每个计划在 8 到 18 个低梯度点用离子室测量。这些计划中测量的每个点都与计划系统预测的剂量相差在 4%以内，并且计划被认为可以临床使用。然后，每个计划都使用激进的剂量约束进行重新优化，以便新的治疗场比原始计划的调制程度更高。非常复杂的场可以计算得不太准确，并且在圆柱形体模中对这些计划的离子室测量证实了明显的剂量学误差-每个计划中的几个测量点与计算剂量相差超过 4%，最大单个偏差为 10.6%。这三个计划被认为不适合临床使用。然后，使用两种单野平面剂量测定方法对所有六个计划（三个可接受，三个不可接受）进行分析：电子射野影像装置（EPID）和离子室阵列的电子门控成像。使用 2%/2mm 距离符合（DTA）和 3%/3mm DTA 标准对每组平面测量进行伽马分析，试图确定一个伽马分析阈值，该阈值可以将有缺陷的计划与可接受的计划区分开来。
使用 EPID 和 2%/2mm DTA 标准，可接受的 IMRT 计划的 88.2%至 92.8%的像素通过伽马分析，不可接受的 IMRT 计划的 87.5%至 91.9%通过伽马分析。使用离子室阵列和 2%/2mm DTA 标准，可接受计划中的 92.4%至 94.9%的点通过伽马分析，而不可接受计划中的 86.8%至 98.3%的点通过伽马分析。当伽马标准扩展到 3%/3mm DTA 时，可接受计划和不可接受计划之间的差异进一步减小。使用两种平面剂量计以及两种伽马标准，发现通过伽马分析的像素分数是剂量学准确性的一个很差的预测指标。距离符合，伽马函数的一个组成部分，在复合计划中有临床相关性，但当应用于单个高度调制的场时，它可能会掩盖重要的剂量学误差。虽然单野平面剂量测定可能是有效的 QA 方案的一个方面，但单野测量的伽马分析对整个计划的重要剂量学不准确不敏感。
