First Clinical Experience with All Four Comprehensive Motion Management Gating Strategies In a 1.5 T MR-Linac In Treatment of a Heart-Proximal Lung Lesion
Abstract
Purpose
Magnetic resonance-guided radiotherapy (MRgRT) with a 1.5 T MR-Linac enables real-time tumor tracking but poses unique challenges for lung targets near the heart, where cardiac and respiratory motion coexist. This study reports the first patient treated with all four Comprehensive Motion Management (CMM) gating strategies and evaluates their performance in addressing complex motion uncertainties.
Methods
A 53-year-old female with secondary malignant lymph node metastases adjacent to the bronchial tree and great vessels underwent MRgRT (60 Gy/15 fractions) using CMM. The GTV was tracked in real-time with a 5 mm PTV margin in all directions except 3 mm posteriorly as the envelope, and the Volumetric Overlapping Criterion (VOICE) was set to 100%. Tumor motion trajectories were quantified via Welch’s spectral analysis to isolate respiratory and cardiac contributions.
Results
Pre-treatment drill session tested three respiratory gating strategies (respiratory, breath-hold, exhale). Without visual biofeedback, breath-hold gating showed the poorest performance (59.3% of time spent in low accuracy). Initial chosen exhale gating was discontinued after treating two fractions due to a low duty cycle caused by cardiac motion and MRI artifacts (pulsation and inflow enhancement). Subsequent treatment used exception gating. Exception gating markedly increased the mean duty cycle from 8.5% to 36.6%, decreased time in low accuracy from 70.4% to 25.1%, and reduced low-quality periods from 35.7% to 5.4%. Retrospective spectral analysis identified dominant respiratory motion (14.6 cycles/min) and significant cardiac motion (60.9 beats/min). Higher respiratory rates positively correlated with improved duty cycle (p<0.001), while increased anterior-posterior motion variability negatively impacted duty cycle (p=0.007).
Conclusion
Exception gating without motion prediction overcame cardiac motion interference and enabled beam delivery with a reasonable duty cycle. This comparative dataset demonstrates that gating strategy selection must be individualized based on motion patterns rather than applied uniformly, providing a practical framework for optimizing MRgRT delivery near the heart.