TU‐E‐108‐03: Optimization of the Beam‐On Latency of An Elekta Linac in the Delivery of Gated VMAT

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. The research is partly supported by a grant from Elekta AB.