C Foote*, D McClatchy, J Pursley, Massachusetts General Hospital, Boston, MA

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  C Foote

PO-GePV-T-311 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

ePoster Forums

Purpose: The Varian Ethos system is a closed system comprised of treatment planning, radiotherapy delivery and record and verify components. The linac is a ring delivery system with a single 6MV FFF beam and a stacked and staggered dual layer MLC. The beam model is preconfigured by the vendor and cannot be modified. An independent beam model was commissioned in RayStation 10A (Raysearch Laboratories) to be used for non-adaptive patient treatments and maintain continuity with current clinical workflows.

Methods: Measured output factors, PDDs, and profiles from Varian’s Halcyon representative data were supplemented with additional measured data curves collected at the time of commissioning to create a robust beam model in RayStation10A. MLC parameter measurements were performed in a water tank with a farmer chamber for a series of sweeping-gap and asymmetric sweeping-gap fields. These tests were used to determine tongue-and-groove width, leaf tip width, and transmission for each MLC layer. Beam model verification included ion-chamber open field measurements in water and plan verifications for 17 clinical VMAT plans using microchamber point measurements, ArcCheck and Mobius.

Results: Measurements for T&G width and transmission for both MLC layers matched the beam model and also agreed favorably with results from five other Halcyon machines as presented by Saez et al.. Measured leaf tip width was within 0.08cm of the initial beam model for both MLC layers, for which adjustment of the model did not show any difference in clinical test cases. Beam model verification results showed a mean difference of 0.2% for open field measurements, 0.9% mean difference for microchamber measurements, and mean passing rates for absolute gamma analysis of 3%, 3mm of 98.9% for ArcCheck and 99.9% for a separate Mobius beam model.

Conclusion: It is feasible to use an independent TPS for non-adaptive patients on the Ethos system.

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