Merkel cell carcinoma (MCC) is a rare aggressive skin carcinoma. Approximately 80% of MCC are related to the Merkel Cell Polyomavirus (MCPyV). Although rates of relapse are high, the follow-up strategy lacks consensus. Patients are usually assessed clinically every 3 to 6 months for the first 2-3 years, and every 6 to 12 months thereafter. In the European guidelines, patients with early stages are monitored with clinical examination and ultrasonography of lymph nodes, while whole-body imaging is optional in patients with stage III disease, on a yearly basis for 5 years. Such strategy may prevent the diagnosis of infra-clinical recurrences, whereas patients could still be treated with surgery or radiation therapy. Until 2017, patients with advanced disease were treated with chemotherapies, with no long-term benefit. Immunotherapies with PD-1/PD-L1 inhibitors currently allow durable responses in 50% of such patients. This major change in the management of MCC patients argues for a follow-up strategy that would allow early diagnosis of infra-clinical metastases, when tumoral burden is still low. Given that all patients cannot be monitored by systematic regular imaging, additional non-invasive tools are needed. Blood-based biomarkers as a surrogate of tumor burden are advantageous as they can be repeated over time, providing guidance on when imaging is necessary. The study aims to assess two blood biomarkers, MCPyV T-Ag antibodies and cell-free miR-375, in a prospective fashion from baseline diagnosis, in a cohort of 150 European MCC patients

Inclusion Criteria:

• Patients with a " de novo " diagnosis of MCC, confirmed on histological criteria (neuroendocrine morphology, CK20 staining and/or neuroendocrine and/or SATB2 staining, exclusion of differential diagnosis)
• ≥ 18 years of age
• Written informed consent obtained from the participant

Exclusion Criteria:

• Patients following any measures of legal presentation
• Pregnancy or breastfeeding