Histopathologic Characterization of a Unique Subset of Melanocytic Nevi on the Thigh

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Melanocytic nevi of certain sites including acral, genital, flexural, and auricular can exhibit histologic features usually associated with melanoma. We note a subset of nevi occurring on the thigh, demonstrating similar findings but cannot be clearly classified as dysplastic nevi or malignant melanoma. Twenty-seven histologic parameters were evaluated in 63 pigmented lesions of the thigh (24 with these distinctive features, 27 classic dysplastic nevi, and 12 benign nevi). Most of these unique nevi were junctional (75%) composed predominantly of single cells (67%) without fibroplasia (4%), whereas dysplastic nevi were typically compound (78%) and nested (85%) with fibroplasia (96%). Sixteen (67%) showed pagetoid spread of melanocytes, mostly single cells in the lower half, compared with only one dysplastic nevus. In addition, there was a greater incidence of lentiginous proliferation, lateral extension, focal confluency, and cytologic atypia, but less circumscription, elongation, and bridging of rete ridges. Melanocytic nevi of the thigh can exhibit atypical architectural and cytologic features as seen in other special sites.
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