Proliferative Nodules Arising Within Congenital Melanocytic Nevi: A Histologic, Immunohistochemical, and Molecular Analyses of 43 Cases

The histopathologic interpretation of proliferative nodules (PNs) in congenital melanocytic nevi can present significant challenges as some PNs may exhibit atypical features that make the distinction from melanoma difficult. We compared histologic features, Ki-67%, PHH3, and CD117% expression levels by immunohistochemistry in 18 benign and 25 atypical PNs (from 41 patients) with that of background congenital nevi (of these 43 cases), 10 congenital nevi, and 3 dermal melanomas arising in congenital melanocytic lesions. In addition, we evaluated the presence of BRAF, GNAQ, HRAS, KRAS, and NRAS mutations in all groups using the SNaPshot Multiplex System. Follow-up was available on 19 patients (9 benign and 10 atypical PNs) (range, 2 to 20 y; median, 8 y) and all were alive with no evidence of disease. The specific histologic features of atypical PNs, such as sharp demarcation (P