Program Information
Optimization of Dosimetric Leaf Gapvalue Using ArcCHECK and Highly Modulated VMAT Plans
S Yu*, D Rosenzweig , T Barry , M Pacella , The University of Rochester Medical Center, Rochester NY
Presentations
WE-RAM2-GePD-TT-4 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Therapy ePoster Theater
Purpose: To determine the optimal dosimetric leaf gap (DLG) for Eclipse TPS using ArcCHECK QA results.
Methods: ArcCHECK (Sun Nuclear Corp) was commissioned on Varian Trilogy equipped with HDMLC. Ten previously treated VMAT plans using 6MV/6SRS and various sites and modulation levels were selected. ArcCHECK QA results were analyzed with SNC Patient software using Gamma tests with 3%/3mm/10% (GM33) and 2%/2mm/10% (GM22) criteria. For the most modulated plan (highest Modulation Factor (MF) ≡ Total MU / Total Dose(cGy)), three modified plans were generated with different modulation levels to investigate the effects of DLG on ArcCHECK QA results. Various DLG numbers were used in the Eclipse dose prediction, providing a means to optimize agreement with ArcCHECK results.
Results: The effect of varying DLG on ArcCHECK results is more pronounced for highly modulated plans. For the three plans with MF of 4.97, 3.37 and 2.46, the gamma rate using GM33 was 52.4%, 68.5%, and 89.8% respectively, with the initial measured DLG of 0.19cm. The DLG value was then tested using 0.05cm, 0.07cm, and 0.08cm, with corresponding passing rates of 95.8%. 96.5% and 96% for the most modulated plan. DLG values of 0.07cm and 0.08cm were subsequently used to calculate Eclipse dose predictions for all 10 patients. The average results using GM33 is comparable for both DLG values (97.31% vs 97.25%). Setting DLG = 0.08 cm produced higher GM22 results compared to DLG = 0.07cm (93.11% vs. 92.11%). After final adjustment of DLG (0.19cm->0.08cm), the average passing rate improved from 90.82%->97.25% for GM33 and from 78.91%->93.11% for GM22.
Conclusion: ArcCHECK proved to be sensitive for detecting variations in dose distribution calculated with different DLG values. Based on the QA results, the original measured DLG value was adjusted to an optimal DLG value and IMRT QA results were improved, especially for highly modulated plans.
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