What Works and What Breaks: Real-World Performance and Limitations of Comprehensive Motion Management on a 1.5 T MR-Linac
Abstract
Purpose
MRI-based intrafraction motion management is increasingly used during MR-guided adaptive radiotherapy, yet limited data exist on its real-world performance and limitations across disease sites. This study reports institutional experience with Comprehensive Motion Management (CMM), with emphasis on delivery efficiency, common failure modes, and practical lessons learned during routine clinical use.
Methods
Patients treated with daily online adaptive radiotherapy on a 1.5 T MR-Linac with CMM enabled were retrospectively analyzed. Motion was tracked on cine MRI using target or soft-tissue surrogates and predefined tolerance envelopes. System log files were used to quantify delivery efficiency (duty cycle), tracking reliability (tracking success rate), and causes of beam interruption. Motion behavior was summarized by disease sites using baseline shift and intrafraction motion metrics derived from cine MRI. Beam interruptions were classified by underlying cause: motion exceeding tolerance, reduced registration quality, or low prediction accuracy during free breathing.
Results
CMM was successfully implemented in 1192 fractions, with a mean duty cycle of 85.2 ± 15.1%, and performance varied among disease sites. Prostate treatments showed the most stable performance, with the highest duty cycle (90.3%) and tracking success (97.8%), reflecting minimal baseline drift and low intrafraction motion. In contrast, abdominal and thoracic treatments demonstrated reduced efficiency and lower tracking success, with kidney performing worst (83.9%). Beam-hold analysis showed that 62.5% of interruptions were caused by target motion exceeding the tolerance envelope, 17.6% by low prediction accuracy, and 16.8% by reduced registration quality. Upper abdominal and thoracic sites exhibited larger superior–inferior baseline shifts and intrafraction motion, directly contributing to these failures.
Conclusion
CMM integrates seamlessly into MR-guided ART workflows and provides a quantitative framework for intrafraction motion assessment. Site-specific differences in duty cycle, tracking success, and beam-hold mechanisms highlight the dependence of CMM performance on motion characteristics and tracking structure design.