Skin melanoma

The attention paid in the past to hypothetical risk factors for developing a melanoma in the context of a pre-existent naevus, such as particular body area  (acral, genital) or reiterative trauma is no longer accepted.

The only type of lesion which clearly  represents a precursor of melanoma are gigantic melanocytic naevi (size bigger than 20 cm). These lesions show indeed a rate of malignant transformation in 10% of cases.

Congenital naevi of medium size (between 1.5 and 20 cm) have to be considered potential precursors, but to a considerable lower extent, while small congenital naevi (size smaller than 1.5 cm) show a rate of malignant transformation completely overlapping to that of acquired melanocytic nevi.