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The Feasibility of Clinical Implementation of AAPM TG-218 IMRT Measurement-Based Tolerance Limit Recommendations

R Price*, I Veltchev , T Lin , A Eldib , C Ma , Fox Chase Cancer Center, Philadelphia, PA


(Monday, 7/15/2019) 1:15 PM - 1:45 PM

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 ϒ 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 (≥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 ϒ value in each histogram and percentage of pixels exceeding a ϒ 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.


Intensity Modulation, Quality Assurance


TH- External beam- photons: Quality Assurance - IMRT

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