Quantifying Intrafraction Motion and the Impact of Gating for MRI-guided Stereotactic Radiotherapy for Prostate Cancer: Analysis of the MRI arm from the MIRAGE Phase III Randomized Trial

PURPOSE:Real-time intrafraction tracking/gating is an integral component of MRI-guided radiotherapy (MRgRT), and may have contributed to the acute toxicity reduction during prostate stereotactic body radiotherapy (SBRT) observed on the MRgRT-arm of the MIRAGE randomized trial (NCT04384770). Herein we characterized intrafraction prostate motion and assessed gating effectiveness. METHODS:Seventy-nine patients were treated on an MR-LINAC. Real-time cine imaging was acquired at 4Hz in a sagittal plane. If >10% of the prostate area moved outside of a 3mm gating boundary, an automatic beam hold was initiated. An in-house tool was developed to retrospectively extract gating signal for all patients, and identify the tracked prostate in each cine frame for a subgroup of forty patients. The fraction of time the prostate was within the gating window was defined as the gating duty cycle (GDC). RESULTS:A total of 391 treatments from 79 patients were analyzed. Median GDC was 0.974 (interquartile range 0.916-0.983). Fifty (63.2%) and 24 (30.4%) patients had at least 1 fraction with GDC≤0.9 and GDC≤0.8, respectively. Incidence of low GDC fractions among patients appeared stochastic. Patients with minimum GDC<0.8 trended towards more frequent grade 2 genitourinary toxicity compared to those with minimum GDC>0.8 (38% vs. 18%, p=0.065). Prostate intrafraction motion was mostly along the bladder-rectum axis, and predominantly in the superior-anterior direction. Motion in the inferior-posterior direction was associated with significantly higher rate of acute grade 2 genitourinary toxicity (66.7% vs. 13.9%, p=0.001). Gating limited mean prostate motion during treatment delivery in fractions with a GDC<0.9 (<0.8) to 2.9 mm (2.9 mm), versus 4.1 mm (4.7 mm) for ungated motion. CONCLUSION:Fractions with large intrafraction motion were associated with increased toxicity and their occurrence among patients appears stochastic. Real-time tracking/gating effectively mitigated this motion and is likely a major contributing factor of acute toxicity reduction associated with MRgRT.