Rectal cancer is a common cancer with an incidence of approximately 14,000 new cases per year in France. The survival rate is approximately 50% at 5 years, ranging from 90% for patients with localized rectal cancer to 18% for patients with metastases.

Surgery with local or complete exicion is the standard treatment for early stages. The implementation of Total Neoadjuvant Therapy (TNT) for locally advanced stages has led to a significant improvement in the prognosis for rectal cancer. The complete pathological response rate of aproximately 20% after TNT has led to the "Watch and Wait" strategy, which aims to avoid surgery and preserve the rectum, but there is currently no reliable method for identifying responders. Rectal tumors with mismatch repair defiency (MMRd) and/or High microsatellite instability (MSI-H) are considered resistant to neoadjuvant chemotherapy, but those patients can benefit from immunotherapy treatment. Identifying patients who will respond to immunotherapy is crucial for this type of treatment. Pre-existing immune infiltration within the tumor microenvironnement prior to any treatment is an important prognostic factor and could help predict response to immunotherapy, as well as neoadjuvant treatment. Studying the immune microenvironnement and how it changes during different treatments could therefore help identify responders for whom treatment without surgery could be considered in order to avoid the deterioration in quality of life often associated with this procedure in this type of cancer. Finally, analysis of residual tumor cells after chemotherapy and/or chemoradiotherapy could help identify possible mechanisms of resistance to current treatments and develop strategies to counter them.

Inclusion Criteria:

• Male or female aged 18 or over
• Patients who have undergone surgery for rectal cancer since January 1st, 2019
• Histological diagnosis of rectal adenocarcinoma confirmed on a surgical sample

Exclusion Criteria:

• Not applicable