Biologic Gray Zone of Melanocytic Tumors in Reality: Defining 'Non-Conventional' Melanocytic Tumors

The ‘gray zone’ and the ‘borderline malignant’ concepts are widely used in Surgical Pathology because of their considerable explanatory potential; however, they require a rigorous definition, since, as their very name suggests, they move within an ambiguous terrain (between white and black; ‘borderline’ between benign and malignant; and, specifically, ‘borderline’ between nevus and melanoma). Confusion exists between intermediate (borderline) morphology and intermediate (borderline) biology, both ‘intermediates’ being often approached with the same set of histopathological criteria, which, in our opinion, is a conceptual and practical mistake. The concept of morphologically intermediate melanocytic neoplasms is implicit to the assumption that melanomas and nevi are “reciprocal morphological simulators”; the differential diagnosis between couples of simulators is based upon the simultaneous evaluation of a standard set of criteria which are subjectively implemented and evaluated, thereby bearing an inherent diagnostic uncertainty (and, parenthetically, a poor interobserver agreement) in some cases [1]. The concept of biologically intermediate melanocytic tumors is referred to neoplasms which are sticto sensu neither nevi or melanomas and are therefore not evaluable as couples of simulators. These tumors are identified as melanocytomas by the World Health Organization (WHO) [2]; we also define melanocytomas as “non-conventional melanocytic tumors”, in order to underline their peculiar clinicopathological and biological properties [3]. © 2024 Mattioli 1885. All rights reserved.