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Reproducibility of Lung SBRT Delivery On Beam-Matched Truebeam Linear Accelerator: A Multi-Linac Study

K Melville1*, K Yang1, R Patel2,1, S Sharma1, W Jiang2,1, (1) East Carolina University, Greenville, NC,(2) 21st Century Oncology, Greenville, NC


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: review the reproducibility of the delivering quality of lung SBRT (VMAT plans using FFF beams) on TrueBeam (Varian Medical Systems, Palo Alto, CA) linac systems.

Methods: patients with stage I/II NSCLC who received SBRT on TrueBeam linacs in 2019 and 2020 were randomly selected in this study. A total of 19 fields were delivered to the EPIDs attached on 3 beam-matched TrueBeams(TB), which were well maintained and calibrated following the recommendation from TG-142 report. The energy used for all fields is flattening-filter-free 6MV with the dose rate of 1400 MU/Min. Treatment plans delivered to EPIDs were analyzed in Portal Dosimetry (Varian Medical Systems, Palo Alto, CA). 2%/2mm gamma criteria was used to compare the predicted dose (calculated with the same beam model in Eclipse TPS) with the dose delivered to each TB. Maximum Gamma(MG) and Average Gamma(AG) were also compared between TBs in the analysis. A statistical study was performed for the gamma passing rate, MG, and AG.

Results: average difference of the passing rates between TB1 and TB2, TB1 and TB3, and TB2 and TB3 were -0.37%±0.51%, -0.06%±0.52%, and 0.32%±0.56%, respectively. Among 57 comparisons of the passing rates for 19 fields, 71.9% (41/57) of them was within the difference of 0.5%. And the most difference of passing rates we found was 1.5%. The most difference of MG was found between TB1 and TB3(0.18±0.40) and the least difference of MG was found between TB2 and TB3 (0.04±0.31). In addition, the least difference of AG was found between TB1 and TB3(0.01±0.02) and the most difference of AG was found between TB1 and TB2(0.04±0.01).

Conclusion: factory quality control for TBs was well implemented. The delivering quality of lung SBRT using 6MV-FFF beams among TBs in our clinic is reproducible and meet our expectation.

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