Blue nevi and related lesions: a review highlighting atypical and newly described variants, distinguishing features and diagnostic pitfalls.

Blue nevi and related entities are a heterogenous group of congenital and acquired melanocytic tumors that includes established entities such as dendritic ("common") blue nevus and cellular blue nevus, and their numerous clinical and pathologic variants, such as deep penetrating nevus. They share several clinical and morphologic features including their blue tinctorial properties, the presence of a dermal proliferation of spindle, fusiform or ovoid cells, associated melanin pigment (both within the melanocytic tumor cells and also within macrophages) and stromal sclerosis and, at least focal positivity for HMB-45 (Gp100). Some variants, such as deep penetrating nevus, often show considerable variation in nuclear size and shape, and, as a consequence, are at risk of being misdiagnosed as melanoma by those unfamiliar with their characteristic morphologic features. The so-called malignant blue nevus is a controversial term denoting melanomas arising in association with or exhibiting some morphologic similarities to blue nevus. There are also lesions that are probably related to blue nevi, such as the recently described pigmented epithelioid melanocytoma and the neurocristic hamartomas, whose nature, biologic behavior, and relationship to the better established entities remains to be clearly established. This review aims to present a brief overview of these lesions, highlighting their pathologic characteristics, distinguishing features and potential diagnostic pitfalls, with particular emphasis on recently described entities, molecular findings, controversial areas, and approaches to diagnosis.