Optimal gating envelope size for automatic gating in magnetic resonance imaging-guided radiotherapy.

PURPOSE: This study aimed to investigate the optimal gating envelope (GE) size for automatic gating in magnetic resonance imaging-guided radiotherapy (MRIgRT) for liver cancer. METHODS: This study retrospectively analyzed imaging and radiation treatment data from ten patients with liver cancer who underwent MRIgRT. Three different GE sizes, covering 7, 5, and 3 out of 10 respiratory phases, referred to as 7-, 5-, and 3-phase GEs, respectively, were used to assess the gating treatment method for this patient cohort, in addition to the internal target volume-based treatment approach. MRIgRT treatment plans were created for each of the motion management techniques, and the resulting dosimetric quality was compared between the plans. A vendor-provided motion analysis software was used to calculate tumor motion trajectories and treatment efficiency. RESULTS: Compared to the ITV-based treatment plan, the average dose reductions were 8.2 %, 13.1 %, and 15.4 % for 7-, 5-, and 3-phase GEs. Small GE significantly reduced mean liver dose and OAR exposure in patients with a GTV of 50-100 cm3 and a minimum GTV-to-OAR distance of < 1 cm. Conversely, larger GE enabled higher beam-on ratios with acceptable dose constraints in patients with a smaller GTV and a greater anatomical separation. A significant correlation between GTV-to-OAR distance and normalized OAR dose was observed. CONCLUSIONS: This study revealed the clinical potential of individualized gating strategies informed by real-time motion analysis. Personalized GE optimization may improve precision and organ sparing and reduce treatment time in MRIgRT for liver cancer.