Single center interventional study to explore activity of fibroblasts in the tumor and away in the healthy rectal tissue.

Patients included in the trial will have new biopsy performed before starting the Radiation Therapy and Chemotherapy (RT-CT). These biopsies will study the activity of fibroblasts in the tumor and away in the healthy rectal tissue. Within 8 to 10 weeks after the end of RT-CT, patients will have cancer surgery by proctectomy (with complete removal of the meso-rectum). On the resection specimen, new samples will be taken at the level of the tumor and away from the rectum.This study is single center interventional exploratory .

Inclusion Criteria:

1. Middle rectum adenocarcinoma Tumor(T)x Node (N)+ or T3 /T4 N0 or N+
2. Classification done on pelvic Magnetic Resonance Imaging (MRI) and rectal ultrasonography
3. Age ≥ 18
4. No contraindication to treatment with capecitabine
5. Able to receive radiotherapy 50 Grays in 5 weeks
6. No contraindication for surgery after chemoradiotherapy which will be 8 to 10 weeks after completion of chemoradiotherapy
7. Patient or legal representative provided with information and signature of informed consent

Exclusion Criteria:

1. High rectum adenocarcinoma
2. Contraindication to rectal biopsy: including anti-coagulants or anti-platelet agents (vitamin K antagonists, clopidogrel, aspirin > 160 grams) that cannot be interrupted
3. Pregnant woman or breastfeeding
4. Persons deprived of their liberty, or under guardianship
5. Impossibility of undergoing the trial's medical follow-up for geographical, social or psychological reasons.
6. Patient already included in another therapeutic trial with an experimental medication during the realization of protocol biopsies and surgery
7. Patient not covered by health insurance

Investigators will share de-identified data sets with interested researchers, educators or clinicians. Materials generated under the project will be disseminated in accordance with Institut Curie policies.

Data requests can be submitted starting 9 months after article publication and will be made accessible for up to 12 months.

Access to trial individual participant data can be requested by qualified researchers engaging in independent scientific reserach, and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a data sharing agreement (DSA).