The histopathology of cutaneous malignant melanoma.

The vast majority of cutaneous malignant melanomas (CMM) develop from a proliferation of intraepidermal melanocytes that may progress through radial (including in situ and microinvasive stages) and vertical growth phases (VGP). The currently accepted classification of melanoma is based on the presence (eg, superficial spreading, lentigo maligna, or acral lentiginous melanoma) or absence of a radial growth phase (nodular melanoma). Although not standardized, melanoma may also be classified as to the nature of the VGP, eg, as expansile nodules composed of epithelioid cells, spindle cells, or smaller nevus-like cells supervening on one of the radial growth components previously mentioned, or developing de novo. Less common variants of the VGP include desmoplastic and neurotropic melanomas (often arising with lentiginous melanomas), and the controversial (and perhaps mainly conceptual) entity, minimal deviation melanoma. Other unusual or rare forms of melanoma are malignant blue nevus, balloon cell melanoma, and clear cell sarcoma. An extensive discussion of the differential diagnosis of the various types of melanoma is provided.