(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: HyperArc (HA) is a mono-isocentric VMAT SRS technique that enables the simultaneous irradiation of multiple brain mets on a Varian TrueBeam linac with planning performed using Eclipse. Patient specific QA on HA plans is challenging due to the small sizes of the brain metastases and current limitations in detectors and phantoms. We present a 3-step patient specific QA process that includes three independent checks. The checks include EPID delivery and analysis with Varian PDIP, a dose calculation with Mobius 3D and a film gamma analysis using Gafchromic EBT3 film.
Methods: Varian TrueBeam 2.7, Eclipse 15.5 with a HA planning license. The first step provides a 3 D gamma analysis using Mobius on a radiation oncologist approved patient plan. Next a QA plan is delivered to the EPID and the delivered fluence is analysed using PDIP. A comparison in terms of Calibrated units (CU) between TPS, and the delivered plan is performed for each PTV. Finally, a patient specific plan is delivered to a plastic water phantom with EBT3 film placed at depths corresponding to the individual brain metastases being treated. A complete 3D gamma analysis of the film is performed with our in-house film analysis software.
Results: Mobius gamma analysis performed using the 2%/1.5mm criterion provided a gamma passing rate of at least 97%. The percent CU difference between TPS and EPID was well within 5 % tolerance. Film analysis using a 5%/1mm tolerance for each met resulted in a gamma pass rate of over 95%.
Conclusion: Although Mobius and PDIP provide a rather quick check on the planning and delivery process, we implemented film QA which although more labor intensive can detect errors introduced due to geometrical uncertainties. This novel 3-tiered approach provides a reliable and robust pre-treatment patient specific QA process for HA plans.
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