Fifty-seventh annual meeting of the American association of physicists in medicine
SU-E-T-733: The Effects of Duty Cycle On Gated VMAT Delivery
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.
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.
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.
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.