ROC analysis in patient specific quality assurance

Purpose: This work investigates the use of receiver operating characteristic (ROC) methods inpatient specific IMRT quality assurance (QA) in order to determine unbiased methods to set threshold criteria for gamma-distance to agreement measurements. Methods: A group of 17 prostate plans was delivered as planned while a second group of 17 prostate plans was modified with the introduction of random multileaf collimator (MLC) position errors that are normally distributed with sigma similar to +/- 0.5, +/- 1.0, +/- 2.0, and +/- 3.0 mm (a total of 68 modified plans were created). All plans were evaluated using five different gamma-criteria. ROC methodology was applied by quantifying the fraction of modified plans reported as fail and unmodified plans reported as pass. Results: gamma-based criteria were able to attain nearly 100% sensitivity/specificity in the detection of large random errors (sigma > 3 mm). Sensitivity and specificity decrease rapidly for all gamma-criteria as the size of error to be detected decreases below 2 mm. Predictive power is null with all criteria used in the detection of small MLC errors (sigma < 0.5 mm) Optimal threshold values were established by determining which criteria maximized sensitivity and specificity. For 3%/3 mm gamma-criteria, optimal threshold values range from 92% to 99%, whereas for 2%/2 mm, the range was from 77% to 94%. Conclusions: The optimal threshold values that were determined represent a maximized test sensitivity and specificity and are not subject to any user bias. When applied to the datasets that we studied, our results suggest the use of patient specific QA as a safety tool that can effectively prevent large errors (e.g., sigma > 3 mm) as opposed to a tool to improve the quality of IMRT delivery. (C) 2013 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4795757]