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Optimization of the Beam-On Latency of An Elekta Linac in the Delivery of Gated VMAT

G Cui

G Cui*, D Housley, F Chen, V Mehta, D Shepard, Swedish Cancer Institute, Seattle, WA

TU-E-108-3 Tuesday 2:00PM - 3:50PM Room: 108

Purpose: To optimize the beam-on latency of an Elekta linac in delivering gated VMAT.

Methods: Step-wise optimization was performed using a pre-clinical gating interface developed by Elekta. Phantom studies were performed where surface motion was simulated using a moving chest plate. Two gating windows 77% and 66% around the end of exhalation were used for the gated VMAT deliveries. The treatment plans from three lung patients previously treated with un-gated 6 MV SBRT were used for this evaluation. Baseline of the average beam-on latency of the gated deliveries was first established. Then the average beam-on latencies between a binned dose rate and a continuously variable dose rate (CVDR) deliveries were compared. With the CVDR delivery, further comparison between two different linac gun hold-on times 1.35 s and 6.50 s was performed. The dosimetric accuracy of the gated deliveries was evaluated by comparing the measured with planned coronal dose distributions using gamma-index analyses (3 mm/3% passing rate criteria).

Results: The average beam-on latencies of the gated deliveries with the CVDR were greatly reduced as compared with those of the binned dose rate for gating windows higher than 66%. With the CVDR deliveries, longer gun hold-on times (6.50 s as compared to 1.35 s) made a dramatic improvement on the beam-on latency for gating windows equal to and lower than 66%. There were insignificant dosimetric differences between all the gated deliveries and the un-gated deliveries.

Conclusion: For gated VMAT deliveries using an Elekta linac with the CVDR and the gun-hold time of 6.50 s, the average beam-on latency was reduced to a level (< 0.3 s) that has minimal impact on clinical results. High dosimetric accuracy was demonstrated for as many as 214 beam interruptions during a single 360-degree-arc delivery with gamma-index passing rates were no lower than 99.0% for all tests.

Funding Support, Disclosures, and Conflict of Interest: The research is partly supported by a grant from Elekta AB.

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