"Triggered trap": nevoid malignant melanoma.

Nevoid malignant melanoma probably represents an early form of nodular malignant melanoma in which there is little proliferation of melanocytes in the epidermis and a dermal proliferation that mimics some features of a compound or intradermal nevus, which is difficult to recognize as being due to malignant melanoma. The lesions can contain either small nevoid cells or larger cells that resemble those in Spitz's nevi. Overall the lesions are symmetrical, have minimal proliferation in the epidermis, and often have dispersion of cells at the base. The lesions with small nevoid cells are particularly difficult to distinguish from common intradermal or compound melanocytic nevi. Reactivity of the intradermal component for HMB-45 antigen, without antigen retrieval, or for Ki-67 antigen can show that the dermal cells have an immature phenotype and, in combination with histological criteria, can support a diagnosis of nevoid malignant melanoma.