Room: ePoster Forums
Purpose: TG218 tightens the IMRT QA tolerance by moving from 3%/3mm to 3%/2mm Î³ (denote as Î³3_3 and Î³3_2 respectively). Tomotherapy DQA dose accuracy suffers more from the angular dependency of the detector response as the angular sensor does not work for tomotherapy. In addition, Tomotherapy DQA suffers more from geometric uncertainty as there are couch sags. Both factors would lower the DQA passing rate compared with Linac. The purpose of this study is to investigate the magnitude of this impact.
Methods: 203 patient plans measured with MapCheck from institution A and 101 patient plans measured with ArcCheck from institution B were analyzed using both Î³3_3 and Î³3_2. The cases failed 95% tolerance or 90% action level was investigated. The feature of shifting the measurement to better match the planned dose was studied.
Results: For institution A, 5 cases were below the 90% action level and 15 were below the 95% tolerance using Î³3_3. When Î³3_2 was used, the numbers became 9 and 21. For institution B with ArcCheck, 4 cases were below 95% tolerance with Î³3_3 and 9 cases with Î³3_2. No case failed the 90% action level. Both institutions identified setup error as the biggest culprit and adopted strategies to combat setup uncertainties.
Conclusion: The change in QA tolerance increases the number of cases fail DQA. However, with strategy to reduce the impact of setup errors, the effect is very minor.