Ancient melanocytic nevus.

Ancient melanocytic nevus is an example of a simulator of malignant melanoma, designated ancient because it shares numerous features with ancient schwannoma. Knowledge of the histopathologic characteristics of this benign melanocytic neoplasm should enable pathologists to avoid overdiagnosis of it as melanoma arising in the intradermal portion of a nevus. Ancient nevi are found most commonly on the face of older persons. The neoplasm is usually a dome-shaped, skin-colored or reddish brown papule, usually with features of a Miescher's nevus. Histopathologically, ancient nevi are exoendophytic, mostly intradermal proliferations of two populations of melanocytes: one with large pleomorphic nuclei and the other with small monomorphous ones. The large melanocytes may resemble those of the epithelioid type of Spitz's nevus. A few mitotic figures may be present in a particular section. The epidermis usually is uninvolved, but sometimes there may be a junctional component. Other important findings are degenerative changes that include thrombi, zones of hemorrhage, pseudoangiomatous changes, thick rims of sclerosis around dilated venules, fibrosis, and mucin. Ancient nevi frequently are misdiagnosed as melanoma arising in an intradermal nevus.