A 71-year-old man with spindle-cell neoplasm of unknown origin: a difficult-to-diagnose clear-cell sarcoma.

A patient initially presented in 1988 with a solitary axillary mass, diagnosed as a high-grade neuroendocrine spindle-cell neoplasm; there was no history of a primary cutaneous malignancy. After subsequent development of a pulmonary nodule in 2001 (14-years post initial diagnosis), the case was reviewed and the possibility of metastatic melanoma was raised. The histopathologic and immunohistochemical profile of this melanocytic neoplasm was diagnostic of clear cell sarcoma (CCS) of tendons and aponeuroses, although the differential diagnosis included malignant melanoma, follicular dendritic and interdigitating cell tumors, malignant peripheral nerve sheath tumor, and a category of so-called PEComas. It is the role of pathologists, particularly dermatopathologists, to distinguish CCS from malignant melanoma, and to alert the clinician, because proper diagnosis ultimately influences treatment. We discuss the immunophenotype, differential diagnosis, and molecular signatures of these neoplasms, and review the pertinent literature on these entities.