Malignant peripheral nerve sheath tumor arising in benign ancient schwannoma: A case report with an immunohistochemical study

A rare example of malignant transformation in an ancient schwannoma arisng in the right side of the neck of a 51-year-old man without any clinical man ifestations suggesting neurofibromatosis is described. The tumor, approxima tely 4 cm at its largest dimension, was well circumscribed and had a direct connection with the sympathetic nerve. Microscopically, the central portio n of the tumor showed features of ancient schwannoma characterized by exten sive hyalinization with cystic degeneration, scattered spindle cells with h yperchromatic and tapered nuclei, and some symplastic changes. However, pre dominantly in the outer portion, a proliferation of spindle-shaped cells wi th enlarged nuclei was present. The nuclei of these cells showed irregular contours, coarse granular chromatin texture, and conspicuous nucleoli. Mito tic figures and small necrotic foci with scattered apoptotic bodies were al so seen. Immunohistochemically, S-100 protein was almost negative in areas consisting of overtly atypical cells where the mitotic index evaluated with MIB-1 antibody was 30.5%. In contrast, S-100-positive bland spindle cells were scattered in an extensively hyalinized area with a labeling index less than 3%. P53 protein was strongly positive in atypical spindle cells. Alth ough it is a very uncommon event, definite nuclear atypia, frequent mitotic figures, and the existence of small necrotic foci should be recognized as indicating a diagnosis of malignant degeneration of benign schwannoma. Immu nohistochemistry would be useful as an ancillary technique in such a settin g.