There are only two cases reported in the literature Merkel cell carcinoma in the retroperitoneum.

The most plausible theory is the retroperitoneal mass could be a massively enlarged lymph node where precursor cells became neoplastic.

The less probable theory is the non-described “regression” phenomena of a cutaneous MCC, but we are not found a primary skin lesion.

Preoperative chemotherapy and excision of the primary tumor is the surgical treatment of choice for retroperitoneal MCC.