Program Information
A No-Measurement QA for Single Isocenter Multiple Brain Lesion SRS Plans
J Ho1*, (1) Alta Bates Summit Medical Center, Berkeley, CA
Presentations
SU-I-GPD-T-569 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Linac-based single isocenter SRS treatments for multiple brain lesions have been developed in recent years. However, it is a challenge to perform patient specific plan QA in cases where a large number of small and widely spread-out lesions are involved. We have developed a method to QA SRS plans using a 2nd TPS for verification without complicated and time-consuming patient specific QA measurements.
Methods: SRS plans are prepared for Varian TrueBeam delivery with a HD MLC. BrainLab Elements was used to create dynamic conformal arc plans. The plans were exported into Varian Eclipse TPS, and verification plans were recreated with original Elements MLC files and plan parameters. The Eclipse system was configured and commissioned with mlc defined fields as small as 5 mm in smallest dimension. Dosimetry leak gap (DLG) parameter was optimized to match the small field outputs measured for BrainLab TPS.
Results: Test plans were performed in commissioning, with lesion number up to 8 and size from 0.5 to 3 cm. Eclipse verification plans were fixed with identical MUs as the Element ones, and compared mean and maximum PTV doses for every PTV in the plans. We confirmed that PTV doses were in a good agreement of ≤5% between two TPS plans. We determined that Eclipse could be a secondary verification system with a clinical criterion within 5% for target > 1 cm. For target ≤1.0 cm and located far away from isocenter, the criterion could be loosened to ≤ 7%. Those test plans were also QA measured and confirmed by our normal patient-specific QA measurements described earlier.
Conclusion: Our single isocenter SRS QA method has proven to be practical. It takes a wide acceptable approach to verify every individual target doses without sacrificing accuracy and clinical relevance.
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