Abstract

Blue nevi (BN) are cutaneous pigmented melanocytic lesions in which elongated melanocytes are seen in the dermis. The presence of acquired BN in adults is infrequent and raises the concern for melanoma. BN presenting in extracutaneous locations are unusual but well documented in the oral cavity, conjunctiva, and prostate. Cases of BN presenting in the vulvovaginal area are extremely rare, with very few cases reported in the literature thus far. We present a case of a 35-year-old woman with a large hyperpigmented irregular and non-painful lesion in the vulva. The clinical presentation, location, size, and asymmetry of this lesion is atypical and uncommon. Histopathological evaluation showed a dermal-based highly pigmented spindled melanocytic lesion arranged in nodules without cytologic atypia. The melanocytes are heavily pigmented and embedded in a fibrotic stroma. No mitotic figures were identified. Immunohistochemical stains after bleaching showed that the lesional cells were strongly and diffusely positive for Sox10 and HMB45. Ki67 showed a low proliferation rate (<3%). Beta-catenin and cyclin-D1 were negative. PRAME immunohistochemical stain was negative. Based on morphological and immunohistochemical features, the diagnosis of an atypical mucosal blue nevus with agminated/plaque-type features was given. The differential diagnosis included pigmented epithelioid melanocytoma, deep penetrating blue nevus, and blue nevus-like melanoma. Differentiating this entity from malignant melanocytic lesions is crucial to avoid unnecessary interventions. Considering the rarity of this case, future molecular and genetic studies could provide valuable insights and may be useful in differentiating atypical blue nevus from malignant blue nevus.

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