Enhanced analysis of gating latency in 0.35T MR-linac through innovative time synchronization of a motion phantom and...

This study aims to evaluate how different gantry angles, breathing rates (BPM), cine image speeds, and tracking algorithms affect beam on/off latency and the subsequent impact on target dose for a 0.35T MR-Linac with a 6 MV FFF beam. The investigation incorporates an image-based MRI4D modus QA motion phantom (MQA) and a measurement-based plastic scintillation detector (PSD).

The MQA's target was customized with an insertion for a 1 mm PSD from BluePhysics. Both the PSD and the MQA were simultaneously synchronized to the Linac to capture latency signals. A plan was created in the ViewRay TPS to deliver dose to the target at three gantry angles (0°, 120°, and 240°). Each gantry angle was evaluated at three breathing rates (10, 12, and 15 BPM). The study also examined two imaging speeds (4 and 8 FPS) and four tracking algorithms.

Across all configurations at 4 FPS, the overall mean beam-on latency was 0.339 ± 0.06 s from the PSD and 0.318 ± 0.06 s from the MQA, whereas at 8 FPS it was 0.630 ± 0.07 s (PSD) and 0.609 ± 0.07 s (MQA). Conversely, the overall mean beam-off latency at 4 FPS was 0.153 ± 0.03 s (PSD) and 0.124 ± 0.03 s (MQA), while at 8 FPS it was 0.121 ± 0.06 s (PSD) and 0.205 ± 0.04 s (MQA). The overall mean difference between gating and non-gating doses was an increase of 12.050 ± 9.2 cGy at 4 FPS and 14.044 ± 7.4 cGy at 8 FPS.

This comprehensive study underscores the significant influence of gantry angle, breathing rate, cine imaging speed, and tracking algorithms on latency and dose delivery accuracy in a 0.35T MR-Linac.