Abstract

Plaque-type blue nevi (PTBN) are rare melanocytic neoplasms with a poorly understood clinical course. This study aims to better define the natural history of PTBN, including the risk of transformation to melanoma. Seven PTBN were retrieved from departmental and referral files. Hematoxylin-eosin-stained sections and immunohistochemistry for S100, SOX10, Melan A and HMB45 were assessed. Clinical presenting features and follow-up were recorded. The tumors presented at birth or in adolescence, arising as pigmented plaques on the scalp, cheek, trunk, gluteal cleft, or upper arm. Most patients presented to the medical system as adults (median age, 35 years; range, 16-70) with a female to male ratio of 2.5:1. Histologically, the tumors showed a diffuse dermal growth of pigmented dendritic cells with admixed ovoid spindle cells. In areas, cellular nodules of uniform plump spindle cells with fascicular growth were noted. Involvement of subcutaneous fat and deep fascia was seen in four tumors. Immunohistochemistry showed strong staining for Melan A and HMB45, with weaker staining for SOX10 and S100. One tumor showed malignant transformation characterized by an expansile growth of pleomorphic epithelioid melanocytes with tumor necrosis. Clinical follow-up was available for six patients. Five patients were alive with no evidence of metastasis after a median follow-up of 120 months (range, 8–132 months). The patient with malignant transformation of the PTBN developed inguinal lymph node metastases but is alive with no evidence of disease after 31 months following wide local excision, lymph node dissection, and pembrolizumab therapy. The diagnosis of plaque-type blue nevus is challenging and requires close clinical correlation. Careful clinical monitoring for possible malignant transformation is advisable.

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