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Case Reports

Abstract

Deep penetrating nevi (DPN) are rare melanocytic nevi, that can be very difficult to differentiate from melanoma because DPN can show very atypical features. DPN can invade lymph nodes and can progress to melanoma. Molecular studies are commonly used to distinguish DPN from DPN-like melanoma. We present a case of a 2-month-old female with a mole on the forehead present since birth. On physical examination, there was a 3 x 4 mm irregular brown and dark brown/black pigmented papule in the right temporal scalp. Microscopic examination revealed a combined pattern melanocytic neoplasm composed of conventional nevomelanocytes intermixed with aggregates of epithelioid melanocytes with more pigmented cytoplasm. These nests of atypical, pigmented melanocytes were arranged in a plexiform pattern aggregating along adnexa and the neurovascular bundle. The nuclear atypia varied from moderate to severe and the mitotic count was focally at 4/mm2. Immunohistochemical stains showed the melanocytic proliferation was diffusely positive for HMB45, SOX10, and MART-1 with a Ki-67 proliferation rate of approximately 5%. Expression of p16 was lost. Furthermore, molecular analysis showed the presence of a BRAFV600E and CTNNB1 pathogenic variants consistent with a diagnosis of a deep penetrating nevus. CTNNB1 mutation is found in almost all cases of DPN, and BRAFV600 is the most common concurrent activating mutation. When next-generation sequencing is not available, complete excision with wide and clear margins is recommended. Sentinel lymph node biopsy is controversial. We share this case to report this atypical case and to emphasize the importance of molecular studies for definitive diagnosis.