The TNTRect trial is a prospective observational study that will evaluate the outcome of MR-guided stereotactic body radiotherapy (SBRT) with a simultaneous integrated boost in a hypofractionated treatment of rectum cancer. Patients will be treated in 5 daily fractions of 5 Gy within an overall treatment time (OTT) of 5 days. A simultaneous integrated boost (SIB) till 30Gy will be delivered to the gross tumor volume. Patients will be treated with daily adaptive radiotherapy and online tumor gating on the MRIdian system (ViewRay Inc.). The aim of the study is to improve the complete clinical response rate to offer more patients an organ preserving approach. The primary endpoint is patient response to the treatment, assessed by endoscopy and MRI, or by medical pathology reports after potential resection was performed. As secondary endpoints local control, disease-free survival, overall survival and the patient's quality of life \& hospital anxiety and depression will be measured.