Nevoid malignant melanoma.

Primary cutaneous malignant melanomas with histological features suggestive of benign nevocytic nevi were studied. From a total of about 3,500 cases, 33 patients with sufficient records, histological slides, and follow-up (at least 5 years for disease-free cases) were found; 15 of them had developed metastases, and 8 had died of disseminated melanoma. Some of the following histological characteristics were always observed: cellular atypia, mitoses, infiltration of adnexa, and in the deeper dermis, infiltrative growth, pigmented tumor cells, sharply demarcated tumor nests, and the absence of maturation. Tumor thickness was the most important prognostic criterion. Clinically, the tumors corresponded to nodular and superficial spreading melanomas. It is concluded that, in rare instances, malignant melanomas strongly resemble benign melanocytic/nevocytic nevi. Such cases do not appear to have a lower degree of malignancy and should be treated as normal malignant melanomas.