Nevoid melanoma: a clinicopathological study of seven cases of malignant melanoma mimicking spindle and epithelioid cell nevus and verrucous dermal nevus.

We report seven cases of a distinctive type of malignant melanoma characterized by a deceptively benign histological appearance with an architecture resembling that of benign melanocytic nevi on scanning magnification. Two predominant architectural patterns were observed: a dome-shaped pattern (two specimens) and a verrucoid pattern (five specimens). The specimens with a dome-shaped pattern of growth were characterized by a smooth epidermal surface and a proliferation of epithelioid melanoma cells with an inconspicuous intraepidermal component resembling spindle and epithelioid cell nevi (Spitz nevi). Gradual diminution in the size of dermal nests toward the bases of the lesions simulating the maturation phenomenon of benign nevi was observed; however, the dermal organization in cords and strands of melanoma cells and the persistence of cellular atypia extending to the bases of the tumors allowed their recognition as malignant melanomas. On the other hand, the specimens with a verrucoid growth pattern consisted of broad, exophytic tumors with a verrucous epidermal surface resembling that of papillomatous dermal nevi but distinguished from them by the presence of a continuous proliferation of melanocytes along the dermal-epidermal junction and by confluent sheets of melanoma cells in the dermis without evidence of true maturation. Clinical follow-up showed local recurrence in three patients after intervals ranging from 5 months to 5 years and regional metastasis in one patient after 2 years. The lesions described here may constitute a serious pitfall for diagnosis because of their innocent silhouette on scanning magnification and their superficial resemblance to spindle and/or epithelioid cell nevi and benign verrucous melanocytic nevi. Proper attention to cytological detail and subtle architectural features will aid in recognizing this unusual variant of malignant melanoma.