Inclusion Criteria:  ultracentral location of the lung tumor, which is defined as an expected contact or overlap of the planning target volume with the proximal bronchial tree or esophagus at the time of consultation indication for SBRT of the ultracentral pulmonary tumor maximum diameter of the ultracentral pulmonary tumor < 5cm age > 18 years of age Karnofsky Performance Score > 70% (ECOG Score 0 - 2) ability to lie still on the MR-linac table for at least one hour ability to hold one's breath for more than 20 seconds successful completion of MRgRT simulation for women with childbearing potential, adequate contraception. ability of subject to understand character and individual consequences of the clinical trial written informed consent (must be available before enrolment in the trial)  Exclusion Criteria:  refusal of the patients to take part in the study previous radiotherapy of the lung and mediastinum, if previous and current target volumes overlap patients who have not yet recovered from acute toxicities of prior therapies (planned) treatment with vascular endothelial growth factor (VEGF) inhibitors, e.g. Bevacizumab, within the time interval 2 weeks before and 2 weeks after SBRT pregnant or lactating women contraindications against performing MRI scans (pacemakers, other implants making MRI impossible) participation in another competing clinical study or observation period of competing trials