Room: Exhibit Hall | Forum 9
Purpose: To establish robust, time-minimal end-to-end QA protocols for Varianâ€™s TrueBeam to validate geometric and dosimetric accuracy throughout the treatment planning and delivery process for representative SBRT/SRS cases.
Methods: Site-specific clinical datasets were created for spine, lung, and cranial cases with VMAT plans optimized subject to RTOG prescriptions and constraints. The Varian End-to-End Accuracy Phantom was utilized with film and tungsten-BB inserts. Geometric testing involved extensive Winston-Lutz tests for collimator, gantry, couch, and combined angles, in addition to new streamlined tests for site-specific cases. Simulation was performed with a Siemens CT scanner, images were exported to Eclipse for planning, and plans were exported to TrueBeam for delivery. Phantom positioning was determined based on CBCT-CT simulation registration allowing for shifts on a six degrees of freedom couch. MV images were acquired for Winston-Lutz tests and treatment plans were delivered to Gafchromic EBT3+ film. Geometric and dosimetric test results were analyzed using RIT.
Results: Representative clinical SBRT cases were created to provide templates for standardized image sets, structure sets, and plans for end-to-end testing. Significant time reductions were obtained during geometric test image acquisition: comprehensive tests required 19.5min whereas site-specific required only 2.5min (spine), 4min (lung), or 5min (brain). Geometric analysis compared radiation field and BB centers to determine potential isocenter offsets. Dosimetric analysis involved gamma analysis of TPS-calculated and measured dose distributions evaluated by 3% dose difference and 1mm DTA. Daily machine output was normalized using daily AP/PA fields for in-phantom film measurements in comparison to AP/PA fields measured for the calibration curve to allow for absolute dosimetry.
Conclusion: Site-specific, user friendly, geometric and dosimetric protocols were created to provide end-to-end tests for typical SRS/SBRT treatment sites and techniques. The standardized templates created allow for efficient yet comprehensive end-to-end testing ideal for plan-specific QA prior to each SRS/SBRT treatment delivery.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by a Varian Research Grant and the AAPM Graduate Fellowship.