Nevoid Malignant Melanoma vs. Melanocytic Nevus

Malignant melanomas that histologically resemble ordinary benign melanocytic nevi remain one of the most difficult diagnoses in dermatopathology. These diagnostically challenging malignant neoplasms have been described in the literature using variable terminology in part due to their low incidence and to the paucity of large inter-institutional studies with long-term clinical follow-up. This chapter highlights the histological and immunohistochemical features that help distinguish nevoid melanomas from ordinary nevi. These features are further discussed in the expanded differential diagnosis with other unusual melanocytic neoplasms (benign and malignant) that also must be considered.