Room: Track 3
Purpose: Cardiac motion induces position uncertainties for radiotherapy treatment targets in the thorax and upper abdomen. Prospective cardiac gating has long been applied to acquire images during selected phases of the heartbeat in cardiac MRI (CMR). Inspired by CMR, we propose cardiac gating as a new paradigm for real-time adaptive radiotherapy. In this study, we characterized the performance of a cardiac gating demonstrator for the Elekta Unity MR-linac (Elekta AB, Stockholm, Sweden).
Methods: A gantry-mounted experimental assembly consisting of an optical relay controlled by a Raspberry Pi 4 (RPi) computer, allowed us to gate the treatment by modulating the pulse-repetition frequency of the linac. The Quasar MRI4D phantom was deployed to mimic regular (60 bpm) or irregular (50-70 bpm) cardiac motion (cos^4, 10 mm peak-to-peak amplitude). The phantom-reported motion was continuously streamed to the RPi using a WiFi signal link. Treatment delivery was limited to the most mechanically still portion of the cardiac cycle using a 3.8 mm gating window. A stereotactic arrhythmia radioablation (STAR) treatment plan was created (1x25 Gy, 15 beams). The dose delivery for different motion scenarios was measured using EBT-XD film. The signal transfer and gating latencies were quantified.
Results: The 5th-95th percentiles of measured latencies were: a) 1.4-2.5 ms for the WiFi signal transfer, and b) 1.1-6.8 ms for the gating latency of the gun. The treatment times were 17.0 minutes and 29.8 minutes for the non-gated and gated deliveries, respectively. For a cranial-caudal line profile through the films, the gated deliveries agreed with the static delivery within 0.6 mm for the 10 and 30 Gy isolines. For non-gated delivery, the 10 (30) Gy isoline widened by 2.6 (-2.2) mm relative to the static delivery.
Conclusion: The Unity MR-linac has gating capabilities on a millisecond timescale that can help mitigate the effect of cardiac motion.
Funding Support, Disclosures, and Conflict of Interest: MF Fast acknowledges funding by the Dutch Research Council (NWO) through project #17515 (BREATHE EASY).