Diagnostic challenges of metastatic spindle cell melanoma on fine-needle aspiration specimens

BACKGROUND. Spindle cell melanoma is a morphologic variant of melanoma that can be difficult to diagnose on specimens obtained via fine-needle aspiration (FNA). Published cytology studies concerning this entity were based for the most part on small series. In the current study, a large series of metastatic spindle cell melanoma is described and the diagnostic pitfalls present in FNA samples addressed. METHODS. The authors retrospectively reviewed the cytologic features of 81 metastatic spindle cell melanoma specimens obtained from 67 patients. Corresponding primary tumors or metastatic tumors taken elsewhere from the same patient were also evaluated. RESULTS. The cytologic smears were mostly cellular and comprised of predominantly spindle tumor cells that frequently formed cohesive fascicles or whorls intermingled with scattered epithelioid tumor cells. The classic cytologic characteristics of conventional melanoma (predominantly dyshesive cellular distribution, cytoplasmic melanin pigments, intranuclear pseudoinclusions, macronucleoli, and binucleation or multinucleation) were noted infrequently or, if present, were more readily found in coexisting epithelioid cells. Remarkably, 9% of the cases failed to demonstrate any of the above classic characteristics. In addition, spindle cells demonstrated a wide range of cytologic atypia, from deceptively bland cells resembling reactive fibroblasts to those indistinguishable from pleomorphic high-grade sarcomatous neoplasms. When the morphologic features were compared with those of the primary tumor or metastatic melanoma taken elsewhere from the same patient, cell type discrepancy was found in 20% of the cases in that the previous counterparts demonstrated the epithelioid cell type. Spindle cells also tended to lose immunoexpression of melanoma markers. CONCLUSIONS. Spindle cell melanoma infrequently demonstrates the diagnostic cytologic features and immunoreactivity of conventional melanoma. Varying degrees of cytologic atypia and possible cell type differences from the primary counterpart or metastatic melanoma occurring elsewhere are additional sources of diagnostic challenges, especially in the metastatic setting. Familiarity with cytologic features, combined with clinical and immunoperoxidase findings, is required to avoid misinterpretation.