Rapid perinatal growth mimicking malignant transformation in a giant congenital melanocytic nevus.

Transformation to malignant melanoma in a giant congenital melanocytic nevus observed on the right limb of a 3,300-g newborn boy was strongly suggested by the histologic features of its ulcerated and papular areas: atypical melanocytes, irregular melanin distribution, many mitotic figures, "pagetoid" invasion of the dermis, and destruction of the rete ridges. Electron microscopy, too, showed that the atypical melanocytes had irregularly shaped and folded nuclei, one or more nucleoli, and a cytoplasm rich in organelles and polymorphous melanosomes. Investigation with a panel of monoclonal antibodies, on the other hand, revealed the antigen phenotype of a proliferative melanocytic lesion unaccompanied by the plain expression of antigens usually observed in malignant melanoma. In addition, the clinical picture during a 2-year follow-up has been free from signs of locoregional and systemic progression.