SU‐E‐T‐733: The Effects of Duty Cycle On Gated VMAT Delivery

Abstract

Purpose:When delivering gated VMAT treatments, the gantry moves back and forth after each beam interruption. There are more beam interruptions with smaller duty cycles. This is a study of the effects of gating and duty cycle on the dose output from the treatment machine.Methods:5 previously‐treated SBRT lung patients with 6X FFF RapidArc plans and 4 previously‐treated lung patients with 6X RapidArc plans are used for this study. The treatment machine is a TrueBeam STx with HD‐MLC. A sinusoidal motion generator is used to gate the beam. A MapCheck, and a 0.6 cc ion chamber sandwiched in solid water phantom are used for dose measurements. The MapCheck is used to measure non‐gated, gated with 50% and 30% duty cycles. The ion chamber is used to measure non‐gated and gated with 30% duty cycles.Results:For the 5 SBRT lung plans with 6X FFF, the average percentage of diodes with dose difference less than 0.1% is 99.5% between non‐gated and 50% duty cycle, and 99.7% between non‐gated and 30% duty cycle. For the 4 lung plans with 6X, the average percentage of diodes with dose difference less than 0.1% is 99.5% between non‐gated and 50% duty cycle, and 98.9% between non‐gated and 30% duty cycle. Using the ion chamber, the measured dose difference between non‐gated and 30% duty cycle is within 0.1% for all 10 SBRT arcs and within 0.2% for all 10 non‐SBRT arcs.Conclusion:On a TrueBeam machine, the VMAT dose delivery difference is within 0.2% between non‐gated and gated treatments. This difference decreases with the increase of duty cycle, also decreases with the decrease of target volume.