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Dosimetric Evaluation of Respiratory-Gated VMAT On a Versa HD Linear Accelerator

J Snyder

J Snyder*, R Flynn , D Hyer , University of Iowa Hospitals and Clinics, Iowa City, IA


SU-I-GPD-T-294 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this work is to provide a framework for commissioning an Anzai respiratory gating system (AZ-733VI) and illustrate the dosimetric challenges of volumetric modulated arc therapy (VMAT) for a traveling waveguide accelerator under gated operation.

Methods: The commissioning measurements performed in this study include a comparison of the central axis dose and lateral profile consistency between gated and non-gated deliveries, quantification of the delay in beam-on or beam-off after the gating signal is sent, and a comparison of gamma passing rates for patient specific VMAT QA with gated and non-gated plan deliveries. All measurements were performed using 6MV, 10MV, 6FFF, and 10FFF energies.

Results: For all energies, the central axis dose constancy between gated and non-gated deliveries was within 0.6% and the agreement for open field profile consistency was 97.8% or greater when evaluated using a percent difference criteria of 1%. An average beam-on delay of between 0.22-0.29 seconds was found, but beam-off delays were not discernable. Initial comparisons of gated and non-gated patient specific plan measurements yielded decreases in pass rates, when evaluated at 3%/3mm, as high as 9% for gated deliveries when compared to non-gated deliveries. Further investigation revealed a gantry position overrun on beam-off during VMAT delivery which resulted in gantry position errors, reducing the agreement between planned and delivered dose distributions. By reducing the dose-rate, which effectively reduces the speed of the gantry, the gamma passing rates of gated and non-gated deliveries became equivalent. An empirical method of calculating the appropriate dose-rate is presented to alleviate this issue.

Conclusion: This work brings to light potential dosimetric inaccuracies of gated VMAT delivery when using maximum dose-rate and high gantry speed. The implementation of a reduced dose rate yielded equivalent gamma passing rates between measured and planned dose distributions for gated and non-gated deliveries.

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