The early and intermediate precursor lesions of tumor progression in the melanocytic system: common acquired nevi and atypical (dysplastic) nevi.

Intermediate lesions of melanocytic tumor progression are potential precursors, simulants, and risk markers of melanoma. The clinical, public health, and biologic significance of intermediate lesions warrants their continued recognition and study, although improved schemata for their clinical and histological coding are needed. Blurred boundaries are inherently problematic to the categorization of lesions occurring along a stepwise pathway of increasing clinical and histological atypia. Nevertheless, the concepts of melanocytic dysplasia and of radial growth phase (in situ and microinvasive) melanoma are important to the classification of intermediate lesions of melanocytic tumor progression. Conceptually, these lesions are clearly separable from early and late lesions and from one another, and there is evidence that criteria distinguishing them can be reproducibly applied. Analysis of these intermediate lesions suggests that they represent responses to events (perhaps mutational) induced by ultraviolet light in constitutionally hypersensitive individuals, supporting epidemiological data that implicate sunlight as an etiologic agent for most melanomas. The continuing rigorous application of the methodologies of epidemiology and basic science to the study of these lesional steps will likely lead to the recognition of biologic markers to better distinguish benign from malignant melanocytic lesions.