First Patient Study with Visual Biofeedback for Gating Delivery Efficiency Improvements on a 1.5 T MR-linac: Biofeedback for MR-linac Gating: Patient Study

Purpose: Breathing-induced motion of abdominothoracic lesions is an important source of uncertainty in radiation therapy. Beam gating improves delivery precision, but lengthens treatment delivery time, which can potentially be counteracted by biofeedback (BF). In this study, we investigated whether providing BF results in higher delivery efficiency and benefits the patient experience. Methods and Materials: This study was conducted on a 1.5 T MR-linac, equipped with cine magnetic resonance imaging-based motion monitoring for gated treatment delivery. BF was displayed on an MRI-compatible monitor, visible via an in-bore mirror assembly. In total, 36 volunteers performed 2 consecutive BF sessions of 5 minutes. Questionnaires were conducted to assess the volunteer experience. In phase 1, 12 healthy volunteers were included to select their preferred graphical user interface (GUI) by choosing between a graph-style GUI and a speedometer-style GUI. In phase 2, 24 patient volunteers were included to either perform a subject-determined breath-hold or a fixed breath-hold, using the preferred GUI from phase 1. Here, the delivery efficiency, that is, the time fraction in the gating window and the system usability scale score were quantified. The gating window was defined as an isotropic 3-mm margin around the motion monitoring structure. Results: Two-thirds of healthy volunteers (n = 8) preferred the graph-style GUI. For patient volunteers, the average estimated delivery efficiency was 64% for subject-determined breath-holds (n = 12) and 63% for fixed breath-holds (n = 12). Patients rated the graph-style BF system with an excellent system usability scale score of 83 (SD = 17). Ninety-six percent of patients agreed that the BF system was easy to use, and 92% agreed that the GUI was clear. Eight patients mentioned feeling less claustrophobic when using BF. Conclusions: BF for gating treatments on the 1.5 T MR-linac facilitated an average efficiency of 63% (SD = 12%) and was highly accepted by patients. Furthermore, performance improvements during sessions in individual patients point toward the importance of dedicated training sessions.