Stereotactic body radiotherapy (SBRT) is the preferred treatment for inoperable early-stage non-small cell lung cancer (NSCLC) and for operable cases when patients decline surgery, offering local control (LC) rates of 80-97%. SBRT is also used in oligometastatic cancer. However, its safety and efficacy for ultra-central lung tumours, near the proximal bronchial tree (PBT) and/or esophagus, remain debated. SBRT targeting these tumours has been associated with higher risk of severe toxicity, including pulmonary hemorrhage and pneumonitis. The term ultra-central lung tumours refers to tumours abutting the PBT, trachea, or esophagus. MR-guided online adaptive radiotherapy has been implemented recently using MR-linac technology, where MR-imaging is combined with linac technology in hybrid systems. These systems offer MR imaging for online adaptive radiotherapy within the treatment room and with the patient in treatment position. Despite results regarding technical and clinical feasibility being promising, the high costs of MR-linac systems and the demands for staffing resources of MR-linacs are limiting a broader implementation of this technology. The MAGELLAN trial is currently investigating MR-linac based SBRT in ultracentral lung tumours to reduce severe toxicity, with outcomes pending. At the Department of Radiation Oncology, University Hospital Zurich, MRI-simulations for RT planning using a dedicated scanner have been performed since 2023 for different tumour entities. We have developed and implemented an MRI-based RT online adaptive re-planning workflow followed by RT delivered at a CBCT-guided, conventional linac. We now aim to expand this MR-informed workflow to online adaptive RT for ultracentral lung tumours. With this study, we will investigate the feasibility of MR-informed, online adaptive SBRT for ultracentral lung tumours using a dedicated MR simulator and treatment delivery using conventional CBCT-linac technology.