Association of early malignant melanoma with nevocytic nevi.

The percentage of histologically associated malignant nevocytic nevi is essential for establishing concepts of histogenesis. The literature abounds with studies of this association. However, the results are conflicting and vary between 4% and 72% of malignant melanomas with associated nevocytic nevi with a maximum frequency between 20% and 30%. These different values could be partially explained by the fact that tumors in more advanced stages might have "overgrown" preexisting nevocytic cells. For this reason, only cases of early stage malignant melanoma (tumor thickness less than or equal to 1.5 mm; level of invasion less than or equal to III) were included in this study (n = 150). Thirty-three (22%) of 150 cases of malignant melanoma were associated with nevocytic nevi. In an additional 6.1% of the cases there was a possible but doubtful association. In 45.4% of the cases the nevus cells could be detected below the tumor, in 18.2% they could be seen at the laterally adjacent sides, and in 36.4% they were found at both sites. Fifty percent showed junctional activity from the nevocytic nevi located to the side of the melanoma. We conclude that the finding of approximately 27% of nevocytic nevi with junctional activity among the nevocytic nevi associated with malignant melanoma could be an indicator that at least some malignant melanomas develop within or next to a preexisting and still proliferating nevocytic nevus.