Per‐beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors

Abstract

Purpose: The purpose of this work is to determine the statistical correlation between per-beam, planar IMRT QA passing rates and several clinically relevant, anatomy-based dose errors for perpatient IMRT QA. The intent is to assess the predictive power of a common conventional IMRT QA performance metric, the Gamma passing rate per beam. Methods: Ninety-six unique data sets were created by inducing four types of dose errors in 24 clinical head and neck IMRT plans, each planned with 6 MV Varian 120-leaf MLC linear accelerators using a commercial treatment planning system and step-and-shoot delivery. The error-free beams/plans were used as "simulated measurements" ͑for generating the IMRT QA dose planes and the anatomy dose metrics͒ to compare to the corresponding data calculated by the error-induced plans. The degree of the induced errors was tuned to mimic IMRT QA passing rates that are commonly achieved using conventional methods. Results: Analysis of clinical metrics ͑parotid mean doses, spinal cord max and D1cc, CTV D95, and larynx mean͒ vs IMRT QA Gamma analysis ͑3%/3 mm, 2/2, 1/1͒ showed that in all cases, there were only weak to moderate correlations ͑range of Pearson's r-values: Ϫ0.295 to 0.653͒. Moreover, the moderate correlations actually had positive Pearson's r-values ͑i.e., clinically relevant metric differences increased with increasing IMRT QA passing rate͒, indicating that some of the largest anatomy-based dose differences occurred in the cases of high IMRT QA passing rates, which may be called "false negatives." The results also show numerous instances of false positives or cases where low IMRT QA passing rates do not imply large errors in anatomy dose metrics. In none of the cases was there correlation consistent with high predictive power of planar IMRT passing rates, i.e., in none of the cases did high IMRT QA Gamma passing rates predict low errors in anatomy dose metrics or vice versa. Conclusions: There is a lack of correlation between conventional IMRT QA performance metrics ͑Gamma passing rates͒ and dose errors in anatomic regions-of-interest. The most common acceptance criteria and published actions levels therefore have insufficient, or at least unproven, predictive power for per-patient IMRT QA.

Evaluation of semiempirical VMAT dose reconstruction on a patient dataset based on biplanar diode array measurements

“…While reliable agreement between the calculated and measured/reconstructed dose in a geometrical phantom is the basis for the dosimetric commissioning of an IMRT system, its value for the meaningful patient‐specific, end‐to‐end testing is less clear. With the prevalent 3% globally normalized dose error and 3 mm DTA threshold criteria in particular, 3 , 4 , 5 y‐analysis passing rates for either per‐beam, single‐plane (6) or quasi‐3D (7) array geometries, had weak — and counter‐intuitive, if any — correlation with the conventional clinical DVH metrics. Instances of the $3\%/3\hspace{0.17em}\text{mm}$ passing rate metric's failure to detect systematic errors are numerous 6 , 7 , 8 , 9 .…”

Sensitivity of array detector measurements in determining shifts of MLC leaf positions

“…Nelms et al 23. simulated four types of beam modeling shifts including MLC transmission factor shifts and beam penumbra shifts.…”

Clinical usefulness of MLCs in robotic radiosurgery systems for prostate SBRT

“…Nelms et al demonstrated that the most common acceptance criteria and published action levels are insufficient because there is a lack of correlation between the conventional IMRT quality assurance performance metrics (e.g., the gamma‐index passing rate) and the dose differences in critical anatomical regions of interest 34. Therefore, to evaluate the possible clinical effects of using different types of collimators on the resulting dose distributions, patient‐specific, anatomy‐based quality assurance is required 35, 36…”