Inclusion Criteria:  18 years or older WHO performance score 0-1. Histopathologically confirmed rectal cancer. Lower border of the tumour located below the sigmoidal take-off as established on MRI of the pelvis.  Confirmed high-risk locally advanced rectal cancer, meeting one of the following imaging based criteria:  Tumour invasion of mesorectal fascia (MRF+) The presence of grade 4 extramural venous invasion (mrEMVI) The presence of tumour deposits (TD) The presence of Extramesorectal lymph nodes with a short-axis size > 7mm (LNN) Resectable disease as determined on magnetic resonance imaging (MRI) or deemed resectable disease after neoadjuvant treatment.  Expected gross incomplete resection with overt tumour remaining in the patient after resection, tumour invasion in the neuroforamina, encasement of the ischiadic nerve and invasion of the cortex from S3 and upwards are considered not resectable • Written informed consent.  Exclusion Criteria:  Evidence of metastatic disease at the moment of inclusion or within six months prior to inclusion except for patients with enlarged iliac or inguinal lymph nodes and aspecific lung noduli. Homozygous DPD (Dihydropyrimidine dehydrogenase) deficiency.  Any chemotherapy within the past 6 months.  o Any contraindication for the planned systemic therapy (e.g. severe allergy, pregnancy, kidney dysfunction and thrombocytopenia), as determined by the medical oncologist.  Radiotherapy in the pelvic area within the past 6 months. Any contraindication for the planned chemoradiotherapy (e.g. severe allergy to the chemotherapy agent or no possibility to receive radiotherapy), as determined by the medical oncologist and/or radiation oncologist. Any contraindication to undergo surgery, as determined by the surgeon and/or anaesthesiologist. Concurrent malignancies that interfere with the planned study treatment or the prognosis of the resected tumour.