Characterizing the motion management system accuracy on a 1.5T MR-Linac

Motion management techniques have enabled drastic reductions in treatment volume allowing for dose escalation. The superior contrast of MRI combined with the ability to perform cine imaging during treatment enable surrogate-less tracking for gating. The purpose of this work is to commission motion management system with gating capabilities on the 1.5T MR-Linac and to characterize system performance.

Linear accelerator stability during gating was measured for a wide range of duty cycles (16 % - 100 %). Motion tracking accuracy and gating latency were characterized for varying target sizes and breathing rates, and with and without adaptive motion prediction using a commercially available motion phantom. Clinical treatment plans were created for lung, prostate, pancreas, and liver sites, using varied gating strategies and tumor motion amplitudes. Film measurements were performed for each plan and compared to either the treatment planning system (TPS) or a reference film delivery. Finally, end-to-end testing was performed with the motion phantom and film to quantify the delivery accuracy of the system for exception and free-breathing gating strategies.

Beam characteristics were stable for all duty cycles analyzed, with all measurements within 0.6 % of the ungated baseline. Tracking accuracy showed a strong dependence on breathing rate. The system had difficulties tracking a 1 cm target due to through plane motion caused by the helical path of the phantom. Film comparisons for the clinical plans demonstrated average dose differences within 1.8 % and gamma passing rates (3 %/2 mm) >75 % within the 50 % isodose line. The cases with the worst gamma passing rates corresponded to exhale gating strategy where a systematic shift in dose was observed due to tumor motion interplay with the gating envelope. End-to-end testing showed excellent dose agreement (<3 % average dose difference) and a localization accuracy of <1 mm for targets with non-periodic motion and <2 mm for moving targets.

The motion management system on the Elekta Unity provides advanced capabilities for treating moving targets. The results of this work support the ability of the system to deliver accurate radiotherapy plans with reasonable uncertainties. However, as demonstrated by exhale gating results, better understanding of the clinical impacts of these new functionalities may be warranted.