Squamous cell carcinoma of the anal canal is a relatively rare cancer (less than 3% of digestive cancers) but its incidence has been increasing in recent decades, probably because of its association with HPV (human papillomavirus) infection. Its extension is mainly locoregional pelvic by lymphatic route, rarely metastatic. The standard treatment nowadays is radiotherapy combined with chemotherapy for locally advanced tumors (T2 or more corresponding to a size of 2 cm or more, or N+): mitomycin C and 5-FU (or capecitabine). While the 5-year disease control rates are excellent in localized forms, around 80%, for locally advanced tumors, the prognosis is poorer, with only 70% progression-free survival at 3 years in patients treated with radiochemotherapy. In these patients, it seems particularly interesting to understand the mechanisms of tumor resistance to treatments, in order to increase their efficacy and to propose new therapeutic targets.

The microenvironment of solid tumors, which has been extensively studied in the last two decades, is now recognized as a major factor in tumor development and invasion. Immune cells, and more particularly macrophages, represent an essential component of the tumor microenvironment, and constitute a link between innate and adaptive responses. The presence of tumor-associated macrophages (TAMs), and in particular M2 macrophages, with an anti-inflammatory and anti-tumor action (as opposed to M1 macrophages which are on the contrary tumoricidal and pro-inflammatory), has been studied in many cancers, such as head and neck squamous cell carcinoma, hepatocellular carcinoma, cervical squamous cell carcinoma, and non-small cell lung cancer. To investigator's knowledge, it has not been studied in squamous cell carcinoma of the anal canal.

Inclusion Criteria:

• Patient whose age ≥ 18 years
• Patient with locally advanced squamous cell carcinoma of the anal canal for which he/she has received radiotherapy/chemotherapy: tumor > 2cm and/or locoregional lymph node involvement
• Patient whose main treatment (radiotherapy and chemotherapy) was performed at the Paris Saint-Joseph Hospital
• French-speaking patient

Exclusion Criteria:

• Patient with missing or unanalyzable material
• Patient with missing data
• Patient under guardianship or curatorship
• Patient deprived of liberty
• Patient under court protection
• Patient objecting to the use of his data for this research

Patient with locally advanced squamous cell carcinoma of the anal canal for which he/she has received radiotherapy/chemotherapy: tumor > 2cm and/or locoregional lymph node involvement, whose main treatment (radiotherapy and chemotherapy) was performed at the Paris Saint-Joseph Hospital.