Epithelioid and Fusiform Blue Nevus of Chronically Sun-damaged Skin, an Entity Distinct From the Epithelioid Blue Nevus of the Carney Complex

Epithelioid blue nevus (EBN) was first described in patients with Carney complex (CNC) and subsequently shown to also occur sporadically. Over 50% of patients with CNC harbor mutations in the gene PRKAR1A, which codes for protein kinase A regulatory subunit 1 alpha (R1 alpha) involved in the signaling pathway regulating melanogenesis and melanocytic proliferation. Immunohistochemical expression of R1 alpha has been shown to be absent in the majority of pigmented epithelioid melanocytomas and all CNC-associated EBNs but present in melanomas and other melanocytic nevi. We have observed several examples of EBN occurring in chronically sun-damaged (CSD) skin with a predominance of epithelioid morphology but also containing a component of fusiform and conventional blue nevus cells, which we have termed epithelioid and fusiform blue nevus of CSD skin. Several of these cases demonstrated notable pleomorphism and nuclear atypia with rare mitotic activity raising concern for the possibility of melanoma; however, the clinical outcomes, detailed histologic review, and molecular results were most consistent with a benign melanocytic neoplasm. We report our clinical, histopathologic, immunohistochemistry, and fluorescence in situ hybridization experience with this distinct entity of epithelioid and fusiform blue nevus and demonstrate that it is a unique subtype of blue nevus occurring on CSD skin with a higher frequency of an associated conventional blue nevus component compared with EBN and without association with CNC or loss of R1 alpha expression typically found in pigmented epithelioid melanocytoma and CNC-associated EBN. We also postulate that the epithelioid pattern may represent a subclone of the conventional blue nevus component induced by chronic UV damage.