Organ Preservation in Rectal Cancer by Dose Escalated MR Guided Adaptive Radiotherapy

Inclusion Criteria: * Patients with histologically confirmed diagnosis of rectal cancer localized between dentate line and 12 cm from the anocutaneous line * Any MRI staged rectal cancer meeting the following criteria * cT1-cT3 and * cN0 -cN1 and * cM0 * Inclusion of UICC Stage I tumors only if unsuitable for endoscopic resection or if primary surgery would require permanent colostomy or deep anastomosis with expected poor organ function. * Tumor affects less than 70% of the rectal circumference. * Maximum longitudinal extension of the tumor less than 8 cm on MRI * MR-Staging requirements: High-resolution, thin-sliced (i.e. ≤3mm) magnetic resonance imaging (MRI) of the pelvis. * Cross-sectional imaging of the abdomen and chest to exclude distant metastases. * Aged at least 18 years. No upper age limit. * WHO/ECOG Performance Status ≤ 1 * Adequate hematological, hepatic, renal and metabolic function parameters * Informed consent of the patient Exclusion Criteria: * Lower border of the tumor localised more than 12 cm from the anocutaneous line as measured by rigid rectoscopy * Distant metastases * Preexisting fecal incontinence for solid stool * Prior antineoplastic therapy for rectal cancer * Prior radiotherapy of the pelvic region * Subject pregnant or breast feeding, or planning to become pregnant within 6 months after the end of treatment. * Subject (male or female) is not willing to use highly effective methods of contraception (per institutional standard) during treatment and for 6 months (male or female) after the end of treatment (adequate: oral contraceptives, intrauterine device or barrier method in conjunction with spermicidal jelly). * Other concomitant antineoplastic therapy * Serious concurrent diseases, including neurologic or psychiatric disorders (incl. dementia and uncontrolled seizures), active,uncontrolled infections, active, disseminated coagulation disorder * Other primary tumors with an estimated life expectancy of less than three years * Contraindications for treatment with 5-Fluorouracil or Capecitabine * Psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule (these conditions should be discussed with the patient before registration in the trial).