|Diagnostic categories of acquired melanocytic nevi|
Clark nevi are the most common nevi in man and, moreover, are regarded by many authors as the most relevant precursor lesions of melanoma. Clinical, dermoscopic, and histopathologic variants of Clark nevi are protean, and the differentiation of Clark nevi from melanoma in situ and early invasive melanomas is the major challenge in the realm of pigmented skin lesions. Clark nevi have been eponymically named after Wallace H. Clark, Jr., who, in 1978, first drew attention to this particular type of nevus by studying numerous melanocytic nevi in patients with concomitant melanomas. It is fair to say that Clark and coworkers originally meant that this particular type of nevus, called by them dysplastic nevus, actually represents a distinctive precursor lesion of melanoma with special implications on management and treatment of patients bearing these nevi. However, in the scientific community there is no agreement on the nature of Clark nevi, on what criteria are necessary for diagnosis, both clinically and histopathologically, and on the number of lesions which are needed to have a markedly increased risk to develop melanoma.
The well-known term dermal nevus encompasses two clinical, dermoscopic and histopathologic rather distinctive variants of benign melanocytic nevi, namely, Unna nevus (papillomatous dermal nevus) and Miescher nevus (dermal nevus of the face). Clinically, Unna nevus is a soft polypoid or sessile, usually papillomatous lesion frequently located on the trunk, arms, and neck. The clinical features of Miescher nevus are rather firm, brownish to nearly skin-colored, dome-shaped papules that occur mostly on the face. The clinical features of these two common types of benign melanocytic nevi are often quite straightforward, allowing clinical diagnosis at a glance.
Spitz/Reed nevi are well-known simulators of cutaneous melanoma from a clinical, dermoscopic, and histopathologic point of view.