Malignant melanoma arising in a plaque-type blue nevus.

Blue nevus is a type of dermal melanocytosis and has a variety of clinicopathological characteristics. A few particular variants are generally accepted as discrete clinicopathological entities. Plaque-type blue nevus (PTBN) is one of the variants of blue nevus. PTBN presents at birth or arises in early childhood, and shows a combination of features found in common blue nevus and cellular blue nevus. In this report, we describe a malignant melanoma arising in association with a PTBN in a 65-year-old male. The tumor appeared as a well circumscribed but partially infiltrative upper-abdominal mass, and was heavily pigmented and consisted of a spindle or epithelioid highly cellular component with mitotic figures and tumor necroses. In the skin around the tumor, multiple pigmented lesions were scattered around the trunk, and these had been recognized since childhood. Histologically, skin lesions showed sparse and wide distribution of the dendritic melanocytes with dense melanin pigments and melanophages between the collagen bundles from the reticular dermis to the abdominal skeletal muscle. We also discuss the histological features and clinical course in our patient in context with previous related literature.