Small Cell Variant of Metastatic Melanoma: A Mimicker of Lymphoblastic Leukemia/Lymphoma

It is well-known to pathologists that melanoma is "the great mimicker" and can look like anything. Despite this widespread awareness, the diagnosis remains a continuous challenge, especially when a metastatic melanoma with rare morphology is examined. We report a case of a 64-year-old man with a lung mass and right-sided pleural effusion who underwent video-assisted thoracoscopic surgery for pleural decortication. The history of melanoma was not reported to us. Microscopic examination revealed sheets of small round blue cells infiltrating into the adipose tissue in a lace-like pattern mimicking lymphoblastic lymphoma. Immunohistochemical stains for melanocytic markers, including S-100 protein, Mart-1, and HMB-45, highlighted the neoplastic cells. The tumor was also positive for CD56 and CD117, but negative for pancytokeratin, CD45, cytokeratin 8, TTF-1, WT1, CD34, chromogranin, synaptophysin, and neuron-specific enolase. The findings were most consistent with metastatic small cell melanoma, an uncommon variant of melanoma that closely resembles lymphoblastic lymphoma and other malignant small round blue cell tumors. To our knowledge, we are the first to describe a case of metastatic small cell melanoma to the pleura in an adult. Clinical and histological details are provided with a review of the literature.