Monitoring Treatment Response With On-board DWI During Neo-adjuvant Chemo-radiation for Rectal Cancer Using Magnetic Resonance-guided-radiotherapy Systems

Inclusion Criteria: 1. Patient over 18 years old. 2. Diagnosis of histologically confirmed rectal adenocarcinoma for which nRCT has been indicated. 3. Karnofsky Index (KPS) ≥ 70 or ECOG = 0 or 1 4. Signed consent to participation. 5. For women of childbearing age, effective contraception must also be agreed for the duration of treatment. 6. Affiliation to a social security regimen, or beneficiary of such a regimen. Exclusion Criteria: 1. Predominant mucinous component on initial MRI, biopsy and / or definitive pathology. Mucin may lead to overestimation of ADC values on DWIs. 2. Contraindications to the MRI procedure (non compatible pacemaker or other metallic foreign body, severe claustrophobia). 3. Exclusive radiation therapy. 4. Other associated neo-adjuvant treatment. 5. Delayed completion of the surgery (more than approximately 10 weeks after the end of the nRCT) or cancellation of the surgical procedure. 6. Contraindications to capecitabine: Severe hepatic impairment, severe renal impairment (creatinine clearance \<30 ml / min), 7. Known deficiency of Dihydropyrimidine dehydrogenase (DPD), known hypersensitivity to 5-FU / capecitabine and / or its excipients. 8. Participation in a protocol with concurrent treatment. 9. Pregnant or likely to be pregnant (without effective contraception) or breastfeeding 10. Person in emergency situation, person of legal age subject to a legal protection measure, or unable to express his / her consent. 11. Impossibility of attending the medical examination of the test for geographical, social or psychological reasons.