Investigation of Optimal Gating Envelope Size for Automatic Gating in MR-Guided Radiotherapy

Abstract

This study aims to investigate the optimal gating envelope (GE) size for automatic gating in magnetic resonance imaging-guided radiotherapy (MRIgRT) for liver cancer. The effects of gating envelope size on distribution and treatment time efficiency were analyzed using real-time motion data calculated from cine MRI acquired during MRIgRT. Imaging and radiation treatment data from ten liver cancer patients who underwent MRIgRT were retrospectively analyzed. For this patient cohort, in addition to the internal target volume-based treatment approach, a gating treatment approach was evaluated using three different GE sizes covering 7, 5, and 3 out of 10 respiratory phases (referred to as 7-phase, 5-phase, and 3-phase GEs, respectively). MRIgRT treatment plans were created for each of the motion management techniques and the resulting dosimetric quality was compared between the plans. Liver doses were analyzed in relation to the gross tumor volume (GTV) size. Doses to organs-at-risk (OARs), such the stomach, duodenum, and bowel, were evaluated by accounting for a distance between the radiation target and each of the OARs. Tumor motion trajectories were calculated using a rigid registration algorithm implemented in a vendor-provided motion analysis tool for automatic gating. The results revealed a clear trade-off between treatment efficiency and normal tissue sparing depending on GE size. In patients with a GTV of 50-100 cc and a minimal GTV-to-OAR distance (< 1 cm), small GE significantly reduced mean liver dose and OAR exposure. Conversely, in patients with a smaller GTV and a greater anatomical separation, broader GE settings enabled higher beam-on ratios with acceptable dose constraints. A significant correlation between GTV-to-OAR distance and normalized OAR dose was observed, underscoring the importance of patient-specific anatomical assessment in GE size determination. This study highlights the clinical potential of individualized gating strategies informed by real-time motion analysis. Personalized GE optimization may enhance precision, improve organ sparing, and reduce treatment time in MR-guided radiotherapy for liver cancer.