Abstract
Agreement between planned and delivered dose distributions for patient‐specific quality assurance in routine clinical practice is predominantly assessed utilizing the gamma index method. Several reports, however, fundamentally question current IMRT QA practice due to poor sensitivity and specificity of the standard gamma index implementation. An alternative is to employ dose volume histogram (DVH)‐based metrics. An analysis based on the AAPM TG 53 and ESTRO booklet No.7 recommendations for QA of treatment planning systems reveals deficiencies in the current “state of the art” IMRT QA, no matter which metric is selected. The set of IMRT benchmark plans were planned, delivered, and analyzed by following guidance of the AAPM TG 119 report. The recommended point dose and planar dose measurements were obtained using a PinPoint ionization chamber, EDR2 radiographic film, and a 2D ionization chamber array. Gamma index criteria {3%(global),3 mm} and {3%(global),3 mm} were used to assess the agreement between calculated and delivered planar dose distributions. Next, the AAPM TG 53 and ESTRO booklet No.7 recommendations were followed by dividing dose distributions into four distinct regions: the high‐dose (HD) or umbra region, the high‐gradient (HG) or penumbra region, the medium‐dose (MD) region, and the low‐dose (LD) region. A different gamma passing criteria was defined for each region, i.e., a “divide and conquer” (D&C) gamma method was utilized. The D&C gamma analysis was subsequently tested on 50 datasets of previously treated patients. Measured point dose and planar dose distributions compared favorably with TG 119 benchmark data. For all complex tests, the percentage of points passing the conventional {3%(global),3 mm} gamma criteria was 97.2%±3.2% and 95.7%±1.2% for film and 2D ionization chamber array, respectively. By dividing 2D ionization chamber array dose measurements into regions and applying 3 mm isodose point distance and variable local point dose difference criteria of 7%, 15%, 25%, and 40% for HD, HG, MD, and LD regions, respectively, a 93.4%±2.3% gamma passing rate was obtained. Identical criteria applied using the D&C gamma technique on 50 clinical treatment plans resulted in a 97.9%±2.3% gamma passing score. Based on the TG 119 standard, meeting or exceeding the benchmark results would indicate an exemplary IMRT QA program. In contrast to TG 119 analysis, a different scrutiny on the same set of data, which follows the AAPM TG 53 and ESTRO booklet No.7 guidelines, reveals a much poorer agreement between calculated and measured dose distributions with large local point dose differences within different dose regions. This observation may challenge the conventional wisdom that an IMRT QA program is producing acceptable results.PACS number: 87.55.Qr
Highlights
The gamma index evaluation method was introduced in a seminal work by Low et al[1] in 1998
The report presents multi-institutional baseline expectation values based on gamma index analysis using 3 mm isodose point distance and 3% dose difference acceptance criteria
Clinical IMRT QA MatriXX isodose distributions for these 50 patients, all of which previously demonstrated 90% or better gamma index passing rates using {3%, 3 mm} criteria were reanalyzed to determine which variable local point dose difference criteria would yield a set goal of 90% gamma index passing scores
Summary
The gamma index evaluation method was introduced in a seminal work by Low et al[1] in 1998. This method enabled comparison of dose distributions in a quantitative manner by calculating the gamma index, the minimum distance in the normalized dose-distance space. The normalization is performed by dividing every dose and spatial coordinate by user selected dose difference (ΔD cGy) and isodose point distance (Δd mm) criteria respectively, resulting in unitless quantities which can be evaluated simultaneously. The report presents multi-institutional baseline expectation values based on gamma index analysis using 3 mm isodose point distance and 3% dose difference acceptance criteria. The dose denominator for gamma calculations is the percent value of the maximum measurement point, i.e., a global normalization value, not the percent value of the local dose
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