Room: Davidson Ballroom A
Purpose: Respiratory-gated volumetric-modulated arc therapy (VMAT) is being utilized by more clinics to treat moving tumors with greater precision. During gated VMAT, the radiation halts as the target exits the gating window. Concurrently, the gantry slows, stops, and rewinds before the subsequent beam-on. For different breathing rates, this gantry motion delays the beam-on time and reduces the efficiency in overall treatment delivery time, negating the delivery-speed advantages of VMAT. The delivery efficiency was measured for different breathing rates, duty cycles, and gantry speeds to determine the optimal parameters for efficient treatment.
Methods: Three arcs of different gantry speeds (6, 4, 2 degrees/second) were delivered on a Varian TrueBeamTM. Sinusoidal respiratory motion was simulated with the QUASARTM phantom and amplitude gating was performed with the Real Time Position Monitoring system. Arcs were delivered with breathing rates of 10-17.1 breaths/minute and gating widows of 30%-90%. Treatment time was measured and compared to expected delivery time to calculate delivery efficiency.
Results: Plans delivered with faster gantry speeds and breathing periods were the least efficient. For all delivered plans, highest efficiency is achieved with 50-70% duty cycles. Two effects contribute to decreased efficiency: as low as 42%. Firstly, at beam-on, gantry speed and dose rate take time to reach planned values; plans with many beam-off events (faster breathing rates and shorter duty cycles) lose efficiency. Secondly, when the duty cycle is large or respiratory rate is fast, the target has already entered the gating window by the time the gantry has slowed, stopped, and rewound.
Conclusion: For a patients with fast breathing rates the gating window for optimal efficiency should be set between 50-70%. For larger gating windows the gantry motion causes a delay in the beam on time, reducing treatment efficiency and prolonging treatment times.