Inclusion Criteria:  Adenocarcinoma of rectum histologically proven  Location ≤ 10 cm from the anal margin  Stage T3T4, or T2 if low rectal cancer With indication of chemotherapy alone or followed by chemoradiotherapy prior to total mesorectal surgery or transanal surgery, discussed in advance at a multidisciplinary consultation meeting oncology No metastasis Operable patient Patient ≥ 18 years old Patient likely to receive radiation therapy and chemotherapy. No history of pelvic radiotherapy for any reason, no history of chemotherapy Effective contraception for patients of childbearing age: male and non-menopausal women must agree to use two medically validated methods of contraception (one for the patient and the other for the partner) during treatment and less up to 6 months for men and 4 months for women after last treatment Patient information and obtaining free, informed and written consent, signed by the patient and his investigator. Affiliate subject or beneficiary of a social security scheme of a member country of the European Community (article L1121-11 of the Code of Public Health)  Exclusion Criteria:  Tumor of the upper rectum (> 10 cm from the anal margin)  Metastatic disease  . T1 stage tumor  History of pelvic radiotherapy and chemotherapy  Contraindication to chemotherapy and / or radiotherapy  . Insufficient tumoral sample on initial rectal biopsy for performing molecular analysis of tumor by NGS.  Other concomitant cancer or history of cancer other than treated in situ cervix cancer or basal cell or squamous cell carcinoma  Pregnant woman, likely to be pregnant or breastfeeding  Symptomatic cardiac and / or coronary insufficiency Severe renal impairment (creatinine clearance less than 30ml / min) Full or partial deficiency in dihydropyrimidine dehydrogenase (DPD) (uracilemia ≥ 16 ng / ml) Peripheral neuropathy> grade 1 Treatment with St. John's Wort Treatment with yellow fever vaccine Treatment with prophylactic phenytoin Treatment with sorivudine or its chemically analogues related, such as brivudine Active infection or other serious underlying disease likely to prevent the patient from receiving treatment Persons deprived of their liberty, or under measure of judicial protection (curatorship or guardianship) or incapable of giving their consent Impossibility of submitting to the medical follow-up of the test for geographical, social or psychological reasons.