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Conference Proceeding

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Medical Physics


Purpose: Previous studies with sample sizes <400 have indicated that VMAT beam complexity indices may have a relationship with measured QA pass-rates. The aim of this study is to evaluate such relationships on a large scale to further investigate their strength. Methods: Total of 17,273 VMAT QA results across 8 Elekta Agility machines were used in this study. All beams were flattened 6MV or 10MV. Sun Nuclear ArcCheck was used for QA measurements with γ-analysis parameters of 10% threshold, 3% dose difference, 2mm distance to agreement, and 95% passing threshold. A total 214 Beams (1.23%) failed QA. QA distributions in Pinnacle were calculated with 2 degree gantry intervals and a dose grid of 2mm. Four complexity indices (MCSv, MU weighted Segment Area, Segment Area/Perimeter, and Average Leaf Gap) were computed. Spearman’s correlations were evaluated for each, and cut-off values for the indices were sought to indicate values above which no QA failures would be expected. Results: Evaluation of the 4 complexity metrics showed weak correlation with QA pass rates (< ±0.42) with high significance (p < 0.001). When metrics were evaluated independently, cut-off values from correlation plots separated only 0.67%, 0.72%, 2.2%, 6.3% guaranteed passing beams from the total sample, respectively. When evaluating whether anymetric for a given beam exceeded its cut-off, the fraction of guaranteed passing beams rose to only 6.6%. However, when cut-off values for one metric were allowed to be functionally dependent upon another, the fraction of guaranteed passing beams increased. For MU weighted Segment Area andAverage Leaf Gap, a dependent approach separated 19.1% guaranteed passing beams from the sample. Conclusion: Individual complexity metrics have weak predictive utility in assessing plan performance. However, using multiple complexity cut-off functions with dependence upon more than one metric increased the fraction of predicted passing beams by around 300%.





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American Association of Physicists in Medicine 65th Annual Meeting & Exhibition, July 23-27, 2023, Houston, TX