SOLITARY FIBROUS TUMOR - HISTOLOGICAL AND IMMUNOHISTOCHEMICAL SPECTRUM OF BENIGN AND MALIGNANT VARIANTS PRESENTING AT DIFFERENT SITES

Twenty-nine tumors (from 26 patients, including two with recurrent dis ease) diagnosed as solitary fibrous tumor (SFT) of the pleura (n = 23) , mediastinum (n = 4), abdominal cavity (n = 1), and parotid gland (n = 1) were studied immunohistochemically. Three histologically malignan t tumors showed areas of high cellularity and mitotic activity (more t han 4 mitoses/10 high-power fields) with features resembling malignant fibrous histiocytoma, malignant hemangiopericytoma, or fibrosarcoma, together with areas typical of benign solitary fibrous tumor. Formalde hyde-fixed, paraffin-embedded tissues and avidin-biotin-complex immuno staining were used. All of the tumors showed vimentin positivity and d id not stain for cytokeratin, glial fibrillary acidic protein, or musc le cell markers, except for focal desmin reactivity in seven tumors, m ostly seen in frozen sections, and focal keratin reactivity in one his tologically malignant tumor. The neoplastic cells were positive for CD 34 and negative for CD31; these patterns also were seen in the three h istologically malignant cases. In nine of the cases acetone-fixed froz en sections showed variable focal positivity for neurofilament protein s of 68 kd. We conclude that SFT is a neoplasm of fibroblasts/primitiv e mesenchymal cells with features of multidirectional differentiation. We also report the finding of a novel site for SFT, the parotid gland . HUM PATHOL 26:440-449. Copyright (C) 1995 by W.B. Saunders Company