Quantification and Temporal Stability of MR-Guided Radiotherapy Gating System Latency Utilizing a Respiratory Motion Phantom
Abstract
Purpose
Intrafraction motion management is essential to reduce setup uncertainty and prevent dose delivery errors during radiotherapy, particularly for tumors influenced by respiratory motion near the diaphragm. Magnetic resonance guided radiotherapy enables real-time beam gating based on direct visualization of the target. The ViewRay 0.35T MRIdian MR-Linac system offers advantages including superior soft tissue contrast, non-ionizing imaging, and flexible imaging planes. Following the A3i system upgrade, gating latency associated with target motion relative to user-defined boundaries has not been well characterized. This study quantifies beam-on and beam-off gating latency after the upgrade and evaluates its temporal stability.
Methods
Gating latency measurements were acquired over five months post-upgrade using a ModusQA QUASAR respiratory motion phantom. The phantom simulated sinusoidal respiratory motion with 6 breaths per minute and 12mm amplitude. Cine MR imaging was performed using a rigid three-plane Cartesian TRUFI sequence at 4 FPS with 3.5mm spatial resolution. Gating was defined using a 30mm margin in the superior–inferior (y) direction on the coronal plane. Phantom trigger positions were calibrated to correspond to cine-reported target excursions of less than 2% beyond the gating boundary. Beam control and phantom-based trigger signals were recorded simultaneously, and latency was determined by comparing the temporal offset between target motion and beam control waveforms.
Results
The weighted mean beam-on latency was 682.15±258.96ms, while the weighted mean beam-off latency was −436.13±272.39ms. Session-to-session variability was limited, with average beam-on and beam-off latencies differing by no more than 334ms and 224ms, respectively. Observed individual measurements of beam-on latencies ranged from 80ms to 1528ms, and beam-off latencies ranged from −979.5ms to 556ms.
Conclusion
Gating latency following the A3i upgrade was temporally stable and appropriate for clinical intrafraction motion management. Beam-off events consistently occurred prior to target excursion beyond the defined boundary, prioritizing patient safety while slightly reducing treatment efficiency.