Curative Chemoradiation of Low Rectal Cancer

Inclusion Criteria: * Histopathologically verified adenocarcinoma of the rectum * Planned abdominoperineal resection (APR) or ultralow resection * Primary, resectable T1-T3, N0 tumor. N1 nodal disease is acceptable, if the positive lymph nodes are localized to the mesorectum at the level of the tumor. * Distance from anal verge to lower edge of tumor ≤ 6 cm measured by rigid rectoscope * Performance status 0-2 * Patient acceptance to collection of biopsies and blood samples for translational research * Age ≥ 18 years * Normal function of bone marrow as evaluated by * neutrophils ≥ 1.5 x 10\^9/l * thrombocytes ≥ 100 * Normal function of liver * alanine aminotransferase (ALAT) \< 2.5 x upper limit of normal * bilirubin \< 2.5 x upper limit of normal * Kidney function \- Serum creatinine \< 1.5 x upper limit of normal or measured glomerular filtration rate (GFR) \> 30 ml/min * Fertile women must present a negative pregnancy test and use secure contraceptives during and 3 months after treatment. * Written and orally informed consent Exclusion Criteria: * Previous surgical treatment of the present cancer, including transanal excision of tumor. * Other malignant disease within the past five years except basocellular skin cancer and carcinoma in situ cervicis uteri * Distant metastases verified by imaging or biopsy * Previous radiation treatment of the pelvis * Pregnant or breastfeeding women