Extended commissioning and validation of the comprehensive motion management (CMM) system on an Elekta Unity MR-Linac

Abstract

Background The Elekta Unity MR-Linac (MRL), operational at Sir Anthony Mamo Oncology Centre (SAMOC) since July 2024, enables daily adaptive radiotherapy (ART) by adjusting treatment plans to account for tumour and anatomical changes. The Comprehensive Motion Management (CMM) system allows real-time motion tracking and gated beam delivery using different strategies, prompting the need for extended commissioning and validation to characterise performance under varied clinical conditions. Objectives To design and deliver extended commissioning and validation tests on the Elekta CMM system, to quantify its positional and dosimetric accuracy across three selected gating strategies (respiratory, exhale navigated, and non-respiratory), and identify any potential limitations. Research Methodology The MRI4D MODUS QUASARTM phantom was commissioned to verify its positional accuracy, forming the basis for subsequent tests. A number of motion traces were used to challenge the system for the three gating strategies. Selected traces were used to quantify the Anatomic Position Monitoring (APM) positional accuracy, measure beam gating latency for both beam-on/off transitions, and assess the dosimetric impact of these parameters during different strategies. The dosimetric impact of different gating thresholds, volumetric overlapping criterion (VOICE) percentages, and complex waveforms was characterised using EBT4 film dosimetry. Results APM analysis confirmed reliable target localisation within typically <±1 mm across all strategies. In the respiratory strategy, regular waveforms met the manufacturer’s latency specification, while irregular waveforms showed degraded performance. Film measurements for respiratory strategy indicated that regular breathing traces maintained target coverage (−0.40±0.22mm change in prescription isodose), while irregular breathing may compromise dose conformity and reduce clinical target volume (CTV) coverage (−1.22±0.23mm change). A trade-off was evident between gating window and efficiency: tighter gating thresholds (2mm) and VOICE 100% improved coverage (0.25±0.20 and 0.08±0.21mm) compared with 5mm and VOICE 95% (–1.13±0.29 and –1.53±0.21mm), but at the cost of delivery efficiency (3.3-5.5× longer). All gating scenarios retained gamma pass rates >97% (3%/2 mm), but a slight reduction was observed for challenging deliveries. Conclusions and Recommendations The CMM system demonstrated robust performance under regular conditions but limitations where characterised under irregular motion including CTV coverage reduction and may need to be accounted for clinically but further investigation is required. Future work should validate CMM in lung-equivalent 4D phantoms and further assess the exhale-navigated strategy.