A study of eleven cutaneous malignant melanomas in adults with small-cell morphology: emphasis on diagnostic difficulties and unusual features.

Abstract

AIMS: To describe the clinicopathological and immunohistochemical features of cutaneous malignant melanomas with a pure or mixed small-cell pattern in 11 adult patients, and to discuss the diagnostic difficulties encountered. METHODS AND RESULTS: Haematoxylin and eosin-stained sections of each case of cutaneous small-cell malignant melanoma, together with locally recurrent skin lesions and, where available, metastatic deposits, were re-examined. Available immunohistochemical sections were evaluated. Clinical follow-up data were obtained in each case. One patient presented with metastatic disease, the others presented with cutaneous lesions. Suggested initial diagnoses included malignant melanoma, non-Hodgkin's lymphoma, Merkel cell carcinoma and sarcoma. All the tumours were in the vertical growth phase. Nine had a junctional component, often inconspicuous. The lesions showed either a pure small-cell pattern or a mixed pattern with more conventional areas. In one case, there was colonization of a basal cell carcinoma by invasive malignant melanoma. Variable retention of small-cell morphology in local recurrences and metastases was observed, although in some cases more typically pleomorphic cells were present. In the cases tested, there was strong immunostaining for S100 protein and NKI-C3, and variable immunostaining for HMB45 and Melan-A. Non-melanocytic markers were negative. CONCLUSIONS: The possibility of a small-cell malignant melanoma should be considered in the assessment of cutaneous and non-cutaneous small-cell neoplasms. The correct diagnosis requires careful evaluation for junctional activity, melanin production and the use of a panel of melanocytic markers.