FINAL DIAGNOSIS: SOLITARY FIBROUS TUMOR OF THE MENINGES
Solitary fibrous tumor occurs most frequently within the pleura, but has also been identified in other tissues, including peritoneum, kidney, liver, lung, bone, nasopharynx, and the orbit (1). Most are benign, but malignant examples have been reported. The solitary fibrous tumor has only recently been recognized as a dura based neoplasm distinct from fibrous meningioma (2). Eight cases of solitary fibrous tumor of the meninges have been reported to date, five in women and three in men, with ages ranging from 43 to 73 years (2, 3). Clinical presentations, imaging characteristics, and sites of predeliction are all similar to those of meningioma, with tumors reported from the parasagittal region (one), tentorium (two), cerebellopontine angle (two), and spinal canal (two). Tumoral cyst formation was noted in one case (3).
Gross, microscopic, immunohistochemical, and ultrastructural characteristics of solitary fibrous tumor of the meninges are similar to those of solitary fibrous tumor of other tissues. The neoplasm is white/tan, firm, and slightly rubbery, with a smooth, unencapsulated surface. In all reported cases, the tumor has been found to compress surrounding CNS parenchyma without gross invasion, although one case described parenchymal microinvasion (2). The lesion's histological appearance is that of a densely collagenous spindle cell neoplasm containing broad, parallel bands of collagen. Immunohistochemical staining shows strong diffuse cytoplasmic immunopositivity for CD34 (nine of nine) and vimentin (nine of nine) and variable immunopositivity for progesterone (five of six) and estrogen (two of five) receptors. Immunohistochemical stains are negative for EMA (nine of nine), keratin (nine of nine), S100 (nine of nine), desmin (nine of nine), smooth muscle actin (eight of eight), collagen IV (seven of seven), and factor VIII-related-antigen (one of one). Electron microscopic studies describe neoplastic cells with ultrastructural characteristics of fibroblasts. Ultrastructural features associated with smooth muscle or meningothelial differentiation have not been identified. Based on these light and electron microscopic characteristics, as well as its distinct immunohistochemical profile, the solitary fibrous tumor is believed to originate from fibroblasts or primitive mesenchymal cells (1, 2, 3).
The histopathological differential diagnosis of solitary fibrous tumor of the meninges (SFT) includes fibrous meningioma (FM), schwannoma, hemangiopericytoma, leiomyoma, various meningeal sarcomas, and other benign fibrous and fibrohistiocytic lesions of the meninges (4). Unlike SFT, FM shows positive immunoreactivity for EMA and S-100 protein (positive in about 80% of FM) and only a mild, focal positivity, if any, for CD34.
Although an understanding of the behavior of solitary fibrous tumor of the meninges is limited by the small number of reported cases, the evidence gathered thus far indicates that its behavior is much like that of solitary fibrous tumor of other tissues, with a largely benign course (2). Malignant forms of solitary fibrous tumor of pleura, characterized by increased cellularity, atypia, mitotic activity, and necrosis have been reported. Of the eight previously reported cases of solitary fibrous tumor of the meninges, one, which was diagnosed as a low-grade fibrosarcoma, showed brisk mitotic activity (2). The patient received localized radiation therapy and there was no tumor recurrence at 10 years follow-up. Of the remaining seven cases only one recurred. The patient subsequently died of sepsis shortly after a second operation for tumor excision.
Contributed by Reza Yaghmai, M.D.,Ph.D., Nancy C. Karpinski, M.D., Wade H. Wong, M.D., William R. Taylor, M.D.