Performance of Unity 1.5 T MR-Linac Gating System across Anatomical Sites and Motion Management Strategies
Abstract
Purpose
to evaluate performance of Elekta Unity 1.5T MR-Linac Comprehensive Motion Management (CMM) system across multiple gating strategies and anatomical sites: gastrointestinal(GI), genitourinary(GU), and thoracic(lung). This work aims to provide a practical reference for clinical implementation, capabilities and limitations of CMM system in clinically realistic scenarios.
Methods
Acceptance and commissioning of the Unity CMM system were performed using the IBA QUASAR™ Modus programmable respiratory motion phantom. Anatomical Position Monitoring (APM) accuracy and system latency were evaluated with a spherical asymmetric tracking target using vendor-defined workflows. End-to-end dosimetry used three MR-compatible small-volume detectors (Semiflex 3D-MR 31024, PinPoint 3D-MR 31025, MedScint HS-RP200), with gating consistency verified by film (Gafchromic™-EBT4) and a 4-channel MRI4D scintillator array (MedScint). To validate system performance under clinically realistic conditions, site-specific testing was conducted using respiratory motion traces extracted from 20 previously treated patients on the Unity system. These patient-derived traces were converted into phantom-ready waveforms and delivered in clinical treatment mode.
Results
APM met 2mm tolerance in all directions (95th percentile: X:1.85±0.02mm, Y:1.54±0.04mm, Z:0.65±0.17mm). With prediction, beam-on/beam-off latency was within specification (–21.0±4ms; –15.2±5ms); without prediction latency increased to 373.9±19ms and 332.1±15ms. Point-dose agreement was 95% (3%/2mm). Output varied <0.2% and profiles <0.1% from ungated, with no symmetry/flatness loss. Using patient-derived motion data, the APM system demonstrated high spatial precision, with mean 3D tracking errors of 0.227±0.265mm (liver), 0.136±0.095mm (lung), 0.095±0.088mm (prostate), and 0.108±0.114mm (pancreas). Latency performance was consistent across free-breathing and exception-gating scenarios.
Conclusion
Unity CMM system demonstrated high tracking accuracy, latency compliance with motion prediction, and dosimetric stability across gating strategies. Validation using motion traces from real, previously treated patients confirms that system performance observed during commissioning translates to clinically realistic scenarios. These results provide a practical benchmark for MR-guided gating implementation while highlighting the importance of patient-derived data in system validation.