Pleural Metastasis of Merkel Cell Carcinoma.

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer, often challenging to diagnose due to its morphological resemblance to other malignancies. Diagnosis is typically confirmed by immunohistochemistry (IHC), with most cases being positive for both neuroendocrine and epithelial markers, particularly the characteristic dot-like cytoplasmic pattern seen with Cytokeratin 20 (CK20). This case report describes a 68-year-old male with MCC originating in the left elbow, who developed pleural metastasis 10 months after initial diagnosis and 4 months following a pathologic complete response. IHC analysis confirmed MCC in the pleural fluid with positive markers including: CK20, chromogranin, synaptophysin, MCC polyomavirus, and SATB2, the latter emerging as a valuable adjunct in distinguishing MCC from other neuroendocrine tumors.