Spectrum of blue nevus-like lesions, including blue nevus, pigmented epithelioid melanocytoma, and animal-type melanoma

Blue nevus-like lesions constitute a category of melanocytic lesions that are clinically identified by their blue coloration. Histologically, they exhibit two primary features: a dermal location and intense pigmentation. The latest World Health Organization classification categorizes blue melanocytic lesions into benign entities (dermal melanocytoses, blue nevus, and deep penetrating nevus), melanocytic tumors with low-to-intermediate malignant potential (pigmented epithelioid melanocytoma), and malignant lesions (blue nevus-like melanoma and melanoma arising in blue nevi). Clinically, blue nevi are enduring and stable lesions, displaying a structureless blue pigmentation clinically and dermatoscopically, with a straightforward histologic diagnosis. On the contrary, lesions with recent onset and/or rapid growth are more commonly associated with diagnoses falling within the intermediate part of the spectrum or with melanoma. These lesions often present with a blue color along with additional features such as black blotches, irregular vessels, and irregular pigmented globules. They typically emerge de novo without recognizable precursors, and they pose significant challenges for patient management. Melanoma on a blue nevus is an exceedingly rare entity with only a few cases described to date. Histologically, differentiating between lesions with intermediate malignant potential and melanoma is always challenging, necessitating a comprehensive evaluation of all morphologic findings of the lesion.