Keratotic melanocytic nevus: a clinicopathologic and immunohistochemical study

Background: Epidermal hyperplasia in melanocytic nevi is a common but littl e-investigated phenomenon. Methods: We prospectively examined all melanocytic nevi diagnosed in our de partment over an 8-month period, for the criteria of keratotic melanocytic nevus (KMN), namely the presence of marked epidermal hyperplasia with or wi thout horn pseudocyst formation, hyperkeratosis, and papillomatosis. In add ition to routine histologic review, we studied 12 representative cases with immunohistochemistry to examine expression of Ki-67, epidermal growth fact or receptor (EGFR), Bcl-2, and Bar. Results: From a total of 1527 melanocytic nevi, 95 were KMN (prevalence 6%) . The average age was 34 years, with a male:female ratio of 1:2. The predom inant location was the trunk (76%), followed by head and neck (20%), and ex tremities (4%). Clinical diagnoses were atypical nevus (44%), nevus not oth erwise specified (43%), and others including seborrheic keratosis, acrochor don, and basal cell carcinoma. Two KMN were junctional, 44 compound, and 49 intradermal. Twenty-three KMN (24%) had histologic features suggesting con genital onset, and 15 (16%) had mild to moderate dysplastic features. Two c ases demonstrated induction of sebaceous glands. Significantly increased Ki -67 expression was detected in the hyperplastic epidermis, particularly in deeper areas related to keratinous cysts and hair follicles. Bcl-2 and Bar (anti- and pro-apoptosis proteins, respectively) and EGFR were expressed si milarly in both normal and hyperplastic epidermis overlying the KMN. Conclusions: KMN are commonly biopsied skin lesions, mostly located on the trunk. Many such lesions are clinically considered atypical, in contrast to their benign histologic appearance. The epidermal hyperplasia on top of KM N demonstrates increased cellular proliferation, in the context of adequate ly regulated apoptosis and EGFR expression. The cellular proliferation seem s to commence in hair follicles.