Pagetoid melanocytosis. Histologic features in benign and malignant lesions.

Pagetoid melanocytosis (PM), the upward discontinuous extension of melanocytes into the superficial epidermis, although generally considered a histologic feature of malignancy, may be seen in certain benign melanocytic lesions. To formulate the histologic criteria for distinction between benign and malignant PM, we examined 218 melanocytic tumors, including melanomas, Spitz nevi, nevi of palms and soles, pigmented spindle cell nevi, recurrent nevi, vulvar nevi, nevi of infancy and early childhood, and ordinary acquired nevi. We found PM to be present in 96% of melanomas, 38% of Spitz nevi, 61% of nevi of palms and soles, 20% of pigmented spindle cell nevi, 60% of recurrent nevi, 80% of vulvar nevi, and none of the ordinary acquired nevi. All the nevi of infancy and early childhood showed PM, but they had been selected for that feature. In melanomas, PM showed significant cellular atypia (81%), which was extensive and diffuse, and in 13% it extended laterally beyond the underlying junctional component. In the benign lesions, cellular atypia was generally absent, nor was lateral extension present, and PM was usually focal or multifocal rather than diffuse and not extensive. Although PM should be considered a tocsin for malignant melanoma, it may also occur in certain benign melanocytic lesions. Accurate interpretation depends on evaluation of all of the pertinent histologic and clinical findings.