CD99 and cytokeratin-20 in small-cell and basaloid tumors of the skin

Although it is classically a deep soft-tissue tumor of childhood, primitive neuroectodermal tumor (PNET) can occur atomy age and may occasionally invo lve cutaneous sites. Merkel cell carcinoma (MCC) and basaloid neoplasms of cutaneous adnexa are the principal diagnostic alternatives to that tumor. T he common expression of CD99 in PNET and cytokeratin-20 (CK20) in MCC sugge sts that these markers may be of value in this diagnostic setting, but they have not been rigorously examined in other small-cell and basaloid lesions of the skin. Accordingly, we evaluated CD99 and CK20 reactivity in formali n-fixed, paraffin-embedded sections of 30 MCC, five cutaneous metastases of pulmonary small-cell neuroendocrine carcinomas, 10 primary cutaneous adnex al carcinomas with basaloid features, 18 benign basaloid adnexal neoplasms of the skin (nine spiradenomas and nine cylindromas), and two cutaneous PNE Ts, using a standard immunohistologic technique and microwave-mediated epit ope retrieval. Of the 30 MCC, 12 showed crisp membrane staining for CD99. A mong the remaining tumors, only the two PNETs were positive for that marker . Although the majority of MCCs did not label for CD99, the pattern of reac tivity in positive cases was indistinguishable from that observed in PNETs. Eighteen of 27 MCCs that were stained for CK20 were reactive for that prot ein, in contrast to metastatic small cell carcinomas, cutaneous PNETs, and appendageal skin tumors, which were uniformly negative for this marker. How ever, a subset of nine tumors, which were most consistent with MCC on clini cal grounds, was CD99 positive and CK20 negative. Hence, reliance on CD99 a lone as a diagnostic marker for PNET in this context cannot be recommended. Rather, careful assessment of the clinical presentation, together with ext ended immunophenotyping that includes other lineage markers and, when possi ble, cytogenetic analysis for characteristic chromosomal aberrations, remai ns the best means of separating MCC from PNET. Finally, the lack of CD99 re activity in basaloid adnexal neoplasms of the skin suggests a utility in th eir differential diagnosis from cutaneous tumors with neuroendocrine or neu roectodermal differentiation.