The application of immunocytochemistry to cytologic direct smears of metastatic merkel cell carcinoma

Merkel cell carcinoma represents a highly aggressive cutaneous malignancy characterized by regional recurrences, lymph node metastases, distant metastases, and high mortality. As the cytomorphology of Merkel cell carcinoma can be mimicked by other malignancies, especially lymphoma and pulmonary small cell carcinoma, immunocytochemistry is often useful in confirming the diagnosis. Cell blocks, which are traditionally utilized for immunocytochemistry, occasionally exhibit insufficient cellularity. Hence, we prospectively investigated the application of CK20 immunocytochemistry to air‐dried, unstained direct smears in the diagnosis of Merkel cell carcinoma fine needle aspirates (FNAs). Eight consecutive FNAs of Merkel cell carcinoma were prospectively examined in this series; seven (88%) cases exhibited immunoreactivity for CK20 in the tumor cells. The one CK20‐negative Merkel cell carcinoma was immunoreactive for synaptophysin and CD56. This immunophenotype was identical to that of the original primary tumor. For comparison, air‐dried direct smears prepared from three pulmonary small cell carcinoma FNAs were examined by CK20 immunocytochemistry. In all cases, no CK20 immunoreactivity was seen in any of the tumor cells. In conclusion, direct smears represent a feasible and robust source of cellular material for immunocytochemical studies to diagnose Merkel cell carcinoma. This methodology allows the cytologist to confirm on site that material for diagnostic immunocytochemistry is present thereby serving as a safeguard in instances where insufficient cell block cellularity is anticipated or encountered. Diagn. Cytopathol. 2013;41:729–733. © 2013 Wiley Periodicals, Inc.