Malignant blue nevus : improved demonstration of cellular atypicality after bleaching of melanin

We describe a case of malignant blue nevus, the diagnosis of which was greatly facilitated by performing histologic examinations after melanin-bleaching procedures A gradually enlarging, blue-black tumor developed in a 62-year-old Japanese man, on the left mandibular region. Histologically, two cell types were recognized with the formation of some areas of necrosis in the dermis : islands of vacuolated cells containing a few fine melanin granules and large ovoid cells full of coarse melanin granules. These tumor cells invaded the underlying muscle. Those with cytoplasmic vacuolation showed polymorphism and nuclear atypia as well as positive staining for HMB-45 and S-100 protein. Bleaching of melanin revealed that the latter melanophage-like ovoid cells were also positive for these stainings. Based on these histopathological and immunohistochemical findings, despite there being no evidence for any metastasis, we reached a final diagnosis of malignant blue nevus.