NEVOID MALIGNANT-MELANOMA - MORPHOLOGIC PATTERNS AND IMMUNOHISTOCHEMICAL REACTIVITY

The term ''nevoid malignant melanoma'' (nevoid MM) is used here to des cribe rare nodular malignant melanomas that may escape detection in ro utine histological sections due to the lack of a prominent intraepider mal component, sharp lateral circumscription and evidence of partial m aturation with descent in the dermis. Nevoid MM mimic ordinary compoun d or intradermal melanocytic nevi when the melanoma cells are small, o r Spitz's nevi when the cells are large. The patterns of HMB-45 staini ng in 12 nevoid MM were compared with those in 107 melanocytic nevi. H MB-45 staining was strong in the dermal component of the nevoid MM, ev en in the absence of a junctional component. In common acquired and co ngenital nevi, the upper dermal component stained less than the juncti onal component of the lesion. The deepest components of these nevi wer e negative. Spitz nevi and cellular blue nevi had positive dermal cell s, even without a junctional component. Additional staining for a prol iferation marker, such as cyclin (PCNA) or Ki-67 (with the antibody MI B-1), can help further in distinguishing a nevoid MM from a Spitz's ne vus. Melanoma has strong nuclear staining throughout the lesion. In co ntrast, Spitz's nevi have more staining at the top of the lesion than at the bottom. The patterns of HMB-45 and MIB-1 staining can be used a long with standard histologic criteria for the diagnosis of nevoid MM. Clinicopathologic correlation is needed to distinguish some metastati c melanomas from primary nevoid MM.