Purpose: To develop a novel dosimetric measurement to quantify the gating latency of an MR-guided radiotherapy (MRgRT) system.
Methods: Gating latency was quantified based on dosimetric deviation of gated and static measurements. A 0.125 cmÂ³ active volume ion chamber was positioned to move across a high dose gradient region at the edge of a fixed field. Measurements were acquired with and without ion chamber motion through this region. The ion chamber was subject to a cosâ?¶ motion trace (amplitude=Â±20 mm, period=6 second) for the gated delivery. The ratio of gated and static measurements was computed and compared to theoretical calculations of the expected relationship between gating dose and system latency. Theoretical calculations served as a lookup table and were determined from the known motion trace and dosimetric falloff along the direction of motion. The test was investigated for reproducibility as well as sensitivity to setup variability. The system latency was monitored longitudinally over eight months as part of monthly QA of a clinical MRgRT linac that images at 4 Hz.
Results: The calculated relationship between gating dose and expected latency for the motion trace and dosimetric falloff (penumbra=7.6 mm) investigated resembled a quadratic with the equation D_Gating=-3.07x10â?»â?µ*LatencyÂ²+0.0013*Latency+98.93, where latency is in milliseconds and gating dose is in percent. The measured gating dose was 97.3Â±0.8% of the static dose for monthly measurements, corresponding to a latency of 253Â±66 ms (N=5 months). Consecutive measurements demonstrated the reproducibility of the technique within Â±26 ms (N=11). Variability in the ion chamber (Â±5 mm) setup along the gradient of the field resulted in an estimated latency deviation of Â±133 ms (N=11).
Conclusion: The developed dosimetric method provided precise verification of gating system latency. The test serves as an estimate of the average beam-on and beam-off latency while utilizing equipment common to many radiotherapy facilities.
Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees and consulting fees from ViewRay Inc. during conduct of this study. Dr Bayouth reports membership on the scientific advisory board of ViewRay Inc. during conduct of this study.