Melanoma mimicking dermal and Spitz's nevus ("nevoid" melanoma)

We describe two examples of malignant melanoma that present with clinical and histopathologic characteristics resembling the benign acquired dermal nevus and the spindle and epithelioid cell nevus (Spitz's nevus), respectively. Both lesions were present on the trunk of adult patients. The clinical impression in both cases was dermal nevus. Histopathologically, these lesions were fairly well circumscribed and symmetrical; they exhibited an expansile dermal proliferation of atypical nevomelanocytes in nests and fascicles with only minimal intraepidermal involvement. These lesions which we will designate as "nevoid" melanoma can be misinterpreted as benign nevi because of the absence of prominent intraepidermal pagetoid spread and the pattern of apparent dermal maturation at the base of the tumor associated with a gradual diminution of cell size. These features mimic the maturation phenomena in banal dermal nevi and spindle and epithelioid cell nevi. The differential diagnosis includes other types of melanoma, and various benign entities characterized by a predominantly dermal proliferative process, such as deep penetrating nevus and cellular neurothekeoma. The recognition of nevoid melanoma is critical so that patients with these lesions receive appropriate therapy for malignant melanoma.