SU-E-T-163: Reproducibility in the Field of Patient-Specific IMRT QA.

PURPOSE To analyze the reproducibility of patient specific IMRT QA results across five different clinical devices and one in-house detector by quantifying variations in the QA results from delivery-to-delivery (without re-setup, "redelivery") and variations when set-up is repeated ("re-setup"). METHODS Six patient plans were selected from among four treatment sites. Six methods of IMRT QA were chosen: film and cc04 ion chamber in a body phantom, diode array with all fields AP, as well as with fields delivered rotationally (in a MapPhan phantom), and an in-house designed solid water phantom with an insert containing 5 ion chambers that can rotate to eight positions. Each of the six plans was delivered three times. The system was then re-setup and redelivered. This last step was repeated, giving three redelivered plans, and three plans that were delivered under independent set ups. Reproducibility was quantified by the coefficients of variation (CV) of the re-deliveries and the re-setup deliveries. RESULTS Of all detectors investigated, film has the highest CV, with an average CV of 1.56% for re-deliveries, and 2.46% for re-setup. For the re-setup and re-delivery, the lowest average CV's are the AP diode array with 0.25%, and the ion chamber with 0.13%, respectively. Additionally the standard deviation in dose across the ion chambers calculated by the TPS was compared to the standard deviation of the re-setup measurements for the in-house phantom, giving a weak correlation. CONCLUSION Different IMRT QA devices exhibit different reproducibility; however all CV's were lower than 5%, indicating good consistency in measurement. Film's dependence on the processor and choice of normalization point are possible reasons why this dosimetry system had a higher CV. This information will help quantify the confidence of IMRT QA performed in the clinic and provide valuable information on the reproducibility of various IMRT QA devices. 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.