Evaluating Suggested Stricter Gamma Criteria for Linac-Based Patient-Specific Delivery QA in the Conventional and SBRT Environments
PHYSICA MEDICA-EUROPEAN JOURNAL OF MEDICAL PHYSICS（2022）
Fox Chase Canc Ctr
Abstract
Purpose: To evaluate AAPM TG-218 recommended tolerances for IMRT QA for conventional and SBRT delivery. Methods: QA analysis was repeated for 150 IMRT/VMAT patients with varying gamma criteria. True composite delivery was utilized, corrected for detector and output variation. Universal tolerance (TLuniv) and action limits (AL(univ)) were compared with statistical process control (SPC) TLSPC and AL(SPC) values. Analysis was repeated as a function of plan complexity for 250 non-stereotactic body radiotherapy (SBRT) VMAT patients at 3%/2mm and a threshold of 10% and for 75 SBRT VMAT patients at 2%/2 mm and a threshold of 50% with results plotted as a function of PTV volume. Regions of failure were dose-scaled on the planning CT data sets based on delivery results. Results: The IMRT/VMAT TLSPC and AL(SPC) for gamma criteria of 3%/3 mm were 96.5% and 95.6% and for 3%/2 mm were 91.2% and 89.2%, respectively. Correlation with plan complexity for conventional fractionation VMAT was "low" for all sites with pelvis having the highest r value at -0.35. The equivalent SBRT PTV diameter ranged from 2.0 cm to 5.6 cm. Negative low correlation was found for 38 of 75 VMAT cases below AL(univ). Conclusions: The AL(univ) and AL(SPC) are similar for 3%/2 mm. However, our 5% failure rate for AL(univ), may result in treatment start delays approximately 2 times/month, given 40 new cases/month. VMAT QA failure at stricter criteria did not correlate strongly with plan complexity. Site-specific action limits vary less than 3% from the average. SBRT QA results do not strongly correlate with target size over the range studied.
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Key words
Gamma criteria,IMRT,VMAT QA
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