Small cell type malignant melanoma which developed in a 16-year-old female with a congenital dermal nevus and metastasized 12 years after excision.

A case of malignant melanoma arising from a congenital dermal nevus on the right forehead of a 16-year-old female is presented. In the mole, small pigmented nevoid cells gathered around the skin appendages and between the collagen fibers. From the age of 17, a verrucous nevoid melanoma consisting of lymphoblast-like large nevoid cells, which were positive for HMB-45 and had a high Ki-67 index up to 18.7%, gradually increased in size. The melanoma cells vertically invaded the dermis to a depth of 3 mm and radially spread in the papillary dermis. Twelve years after undergoing a wide local resection and additional chemotherapy, metastatic lesions were found in the lung and the anterior mediastinum, which gradually increased in size and caused death a few months later. Metastatic melanoma cells were positive for HMB-45 and had a high Ki-67 index up to 33.7%. Most metastatic melanoma cells were positive for p53 while the primary ones were negative. Deteriorating mutations probably accumulated during the latent period.