[Positive cytokeratin results in malignant melanoma. Pitfall in differential immunohistologic diagnosis of occult neoplasms].

Immunohistology was performed on 84 paraffin-embedded surgical specimens of malignant melanomas (MM) from a total of 74 patients. The series consisted of 62 cutaneous primary tumors and 22 (partly selected) secondary manifestations (9 cutaneous recurrences and 13 metastases). In 4 patients with lymph node MM infiltrates, clinical investigations failed to identify a cutaneous primary tumor. In the primary and secondary manifestations respectively, the following proportions of immunohistologically positive cases were recorded: vimentin 100% each, S100-protein 95% each, NSE 87%/77%, HMB45 97%/64%, NKI/C3 97%/95%, cytokeratins (CK) (antibodies KL1, CAM5.2 and 35 beta H11) 0%/23%. Four of the 5 CK-positive lesions belonged to 3 patients in whom MM had occurred at first or exclusively as a lymph node infiltration. These findings confirm the results of other authors who report that positive staining results for CK can be expected in paraffin sections of secondary manifestations of MM in up to 10% of cases in large, nonselected series. This phenomenon appears, however, to be rare in primary cutaneous MM.