SU‐E‐T‐158: Evaluation of the Sensitivities of Patient Specific IMRT QA Dosimeters

Purpose: To analyze the sensitivity of patient‐specific IMRT QA devices to detecting plan failures when compared to multiple ion chamber measurements. Methods: Four methods of IMRT QA were chosen: film (Kodak EDR2) and ion chamber (Wellhofer cc04) in an I'mRT body phantom, MapCheck2 (Sun Nuclear) with fields delivered AP, and MapCheck2 in the MapPhan phantom with rotational fields. Additionally, an in‐house designed multiple ion chamber phantom with an insert containing 5 ion chambers (Exradin A1SL 0.057cc) that can rotate to eight positions was used as a gold standard. Twenty plans previously failing film and ion chamber IMRT QA at our institution and five passing plans were delivered to all devices. The plans' performance on the in‐house phantom sorted them into true fails and passes. The sensitivity to detect true failures was assessed across devices with acceptance criteria of 3%/3mm and 90% of pixels passing for planar, and 3% dose difference for point measurements. Additionally, specificity was analyzed. Results: The sensitivities among all devices were less than 0.70 (poor), indicating a deficiency in identifying plans that failed versus multiple ion chamber measurements. The specificities were generally high. The AP‐delivered MapCheck has the lowest sensitivity (0.27), while the rotationally‐delivered Mapcheck MapPhan had the lowest specificity (0.70) due the directional dependence of diodes. Film was the most sensitive (0.60) dosimeter under investigation, while the single ion chamber had the highest specificity (1.00). Conclusion: The different sampling methods of each device lead to a difference in the ability to detect true failures as determined by multiple ion chamber readings. The overall low sensitivities among all devices indicates that failing plans are passing, leading to a potential lack of error detection in patients' IMRT plans. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.