Cytodiagnosis of amelanotic metastatic malignant melanoma: an immunocytochemical study

Malignant melanomas are known to present diverse patterns and this can result in considerable difficulties for an interpretation of malignancy type from cytohistologic material. Such difficulties are further compounded, if melanin pigment cannot be demonstrated by conventional histochemical stains; unfortunately many cases do exhibit this feature, especially in its metastases. This study was designed to review cases of amelanotic metastatic malignant melanomas in a variety of cytologic samples which were received from 40 patients with a known history of malignant melanoma. Cytomorphologically, the cases were classified as classical, carcinoma-like, spindle cell type, lymphoma-like, and undifferentiated type (Table I). While in 36 of the cases, the diagnosis of metastases from a melanoma was confirmed based on an immunopositivity to a variety of melanoma markers (Table III), in four of the cases, the immunostaining indicated metastases from another primary source, which was subsequently found. Based on our study, we are of the opinion that an immunologic characterization is useful to conclusively diagnose the majority of cases of metastatic amelanotic malignant melanomas. Furthermore, we feel that a reliance on a single melanoma marker is not justified, and a panel of antibodies should be used to distinguish a metastatic amelanotic malignant melanoma from other metastatic neoplasms.