Melanocytic nevi in histologic association with primary cutaneous melanoma of superficial spreading and nodular types: effect of tumor thickness.

The histologic presence of benign dermal nevus cells in contiguity with primary cutaneous melanoma, as a distinct population separate from malignant melanocytes, was evaluated in a large referral data base. The melanomas were limited to superficial spreading melanoma (SSM) and nodular melanoma (NM). Overall, dermal melanocytic nevi were found associated with 1126 of 1954 primary SSM/NM (57.6%). When the melanomas were stratified by tumor thickness, an inverse relationship between the presence of benign nevus cells and tumor thickness was found: 64.9% of tumors less than 0.76 mm and 64.5% of those between 0.76 and 1.69 mm were associated with dermal nevi, whereas in the thickness range 1.70-3.60 mm, there were 45.6% associated nevi, and in melanomas greater than 3.60 mm, there were only 32.0% noted to have nevus cells. When melanomas were separated by nevus type, it was found that 41% were associated with an acquired pattern nevus, 38% with congenital pattern nevus, and 21% with dysplastic nevus. It may be concluded that 1) the histologic presence of nevus cells is a common event in SSM/NM; 2) the association of melanocytic nevus and melanoma is more easily demonstrated in thinner tumors; and 3) acquired pattern nevi, congenital pattern nevi, and dysplastic nevi are all potential precursors of melanoma.