Room: Exhibit Hall | Forum 4
Purpose: The purpose of this work is to determine the feasibility of implementing the AAPM TG-218 recommended tolerances for IMRT QA analysis.
Methods: Patient-specific ionchamber array-based delivery QA analysis was repeated for 100 previously treated IMRT/VMAT patients with varying Ï’ criteria. True composite delivery, corrected for detector and output variation with angular correction applied to all IMRT cases, was utilized. Universal tolerance (TL) and action limit (AL) passing rates (â‰¥95% and 90%, respectively) were determined following global normalization and a low dose threshold of 10%. Statistical process control (SPC) TL and AL values were determined for comparison. The maximum Ï’ value in each histogram and percentage of pixels exceeding a Ï’ value of 1.5 were evaluated for both 3%/3mm and 3%/2mm and results converted deviations of dose in low-dose gradient or DTA in steep dose gradient region. The average time between QA delivery and patient start was determined.
Results: Average global normalization was 94.96% of the maximum dose in the measurement plane. The percentage of cases failing TL for Ï’ criteria of 3%/3mm and 3%/2mm were 2% and 12% and failing AL were 0% and 3%, respectively. The average maximum Ï’ values for these criteria were 1.43 and 1.69, corresponding to 1.29%/ 1.29mm and 2.07%/1.38mm beyond limits, respectively. The average percentage of pixels exceeding a Ï’ value of 1.5 was 0.03% and 0.18%, respectively. The SPC AL and TL for Ï’ criteria of 3%/3mm and 3%/2mm were 96.54% and 95.63% and 91.20% and 89.17%, respectively. Delivery QA was performed the night prior to treatment start in 37% of cases.
Conclusion: The suggested AL and our SPC AL are similar for Ï’ values of 3%/2mm. Assuming 1300 new cases/year with 70% IMRT/VMAT and delivery QA failing the AL in 3% of cases results in treatment starts being delayed approximately 1 time/month.