Effusion cytology of malignant melanoma. A morphologic and immunocytochemical analysis including application of the MART-1 antibody.

Background: Malignant effusions are complications of metastatic malignant melanoma (MM). Differential diagnosis often involves distinguishing MM from adenocarcinoma and reactive mesothelial cells. Descriptions in the literature of the morphologic and immunocytochemical (IM) staining characteristics of MM in effusions are sparse. A combination of morphology and immunocytochemistry should yield the most accurate diagnostic results. The MART-1 antigen, a transmembrane protein, is specifically expressed in melanocytes and MM. A recently developed monoclonal antibody to the MART-1 antigen may represent a useful marker for the identification of MM in effusions.
Methods: The authors conducted a retrospective review of 32 effusion samples diagnosed as MM. The review consisted of morphologic and IM analyses of the effusion samples with antibodies to MART 1, HMB45, S-100, and cytokeratins (AE1/ AE3). IM stains were performed on cell block or cytospin material, depending on availability. In the morphologic review, emphasis was placed on Diff Quik-stained material, due to its enhanced cytoplasmic volume and detail.
Results: Predominant cytologic features noted were lack of cellular cohesion (in 100% of cases), large eccentric nuclei with prominent nucleoli (in 100%), multinucleation (in 84%), variable cytoplasmic vacuolization (in 75%), pigment (in 72%), and cell-in-cell engulfment (in 47%). All immunoreactive cases with sufficient material stained with at least one of the markers used. Tumor cells were positive with IM stains to MART-1 in 78% of cases, HMB45 in 81%, and S-100 in 81%. Coexpression of MART-1, HMB45, and S-100 was noted in 63% of cases. Of cases that showed expression for only 1 of the 3 antigens, the MART-1 was positive in 1 case, and HMB45 and S-100 were positive in 2 cases each. Three cases showed immunoreactivity for cytokeratins in the melanoma cells.
Conclusions: The diagnosis of MM in effusions can be made reliably through a combination of morphologic and IM features. Differential diagnosis often involves distinguishing MM from adenocarcinoma or reactive mesothelial cells. Cytoplasmic vacuolization, multinucleation, prominent nucleoli, and cell-in-cell engulfment are cytologic features common to all three. The lack of IM staining for cytokeratins alone cannot reliably distinguish MM; 11% of cases showed positive staining with this antibody in the melanoma cells. The use of a panel of antibodies increases the accuracy of diagnosing MM. In this study, MART-1 proved a useful adjunct to the HMB45/S-100/cytokeratin panel for the diagnosis of MM in effusions, staining 78% of the immunoreactive cases, with positivity in 1 case that was negative for HMB45 and S-100.