Amelanotic blue nevus: a variant of blue nevus

Blue nevi are typically heavily melanized. We report a variant of blue nevus that is minimally pigmented. Of the 1,358 blue nevi seen in our laboratory during the last 6 years, 38 (2.7%) were selected that had scant or absent melanin. We refer to these blue nevi as the amelanotic type. Approximately half of the cases in clinical diagnosis were nevus of some type, whereas other differential diagnoses were basal cell carcinoma, dermatofibroma, and lesion. Histologically all specimens were characterized by the spindle-shaped cells seen in blue nevi, but with very little or no obvious melanin. Some lesions were markedly cellular, resembling the features of cellular blue nevus. No hemosiderin was identified on Perls' stain, whereas Fontana-Masson stain was variably positive. Usually there was fibrous stroma. In most cases, the histologic differential diagnosis was dermatofibroma. Other histologic differential diagnoses included amelanotic and/or spindle cell melanoma, dermal Spitz nevus, neurofibroma, and scar. There was no pleomorphism or increased mitotic activity. Evidence of epidermal melanocytic hyperplasia was seen in two cases. Furthermore, the lesions had been present for many years without evidence of recent change. Immunohistochemistry showed all cases to be strongly positive with anti Mel-5 antibody, but only weakly positive or negative with anti S-100 and HMB-45 antibodies. We would like dermatologists and pathologists to be aware of this unusual and uncommon entity.