Malignant melanotic schwannoma or schwannian melanoma?

A 43-year-old man underwent surgical removal of L4-L5 nerve root tumor which bulged to the extra and intradural spaces and extended as a dumbbell through the intervertebral foramen. Histological examination showed pleomorphic tumor cells with heavy melanin pigmentation. Most of the tumor cells were immunohistochemically positive for vimentin, S-100 protein and HMB-45 antigen; basic myelin protein was detectable in single tumor cells. Electron-microscopy revealed melanosomes in different stages of differentiation and some characteristics of Schwann cells, such as redundant basal lamina production. Taken together the tumor showed features of histological malignancy and incomplete schwannian differentiation with considerable overlap between melanoma and malignant melanotic schwannoma.