Solitary Fibrous Tumor of the Orbit
Definition: mesenchymal tumor characterized by fibrosis, branching vasculature, CD 34 positivity.
Incidence/Prevalence: There are only about 60 cases of solitary fibrous tumor reported as so named in the literature. However, this is probably a vast underestimation because these tumors were classified as hemangiopericytomas in the past. Hemangiopericytoma and solitary fibrous tumors are probably related entities. Many of the features of the hemangiopericytoma and solitary fibrous tumor, both clinical and histological, are identical.
Clinical Findings: Most patients present with progressive painless proptosis (~60%). Patients may present with epiphora in about 12%. Men are slightly more affected than women (14:11). The mean age of diagnosis is 43.2 yrs with a range of 14-76. The mass is often extraconal and may be present in the superior medial quadrant.
Radiology: The MRI scan is fairly characteristic with the central nidus of collagen (see below) showing hypodense areas on T1 and T2. The cellular portion of the tumor is isointense compared to grey matter and shows moderate heterogeneity and gadolinium enhancement.
Gross and Histopathology: In the gross and at low magnification there is often a surrounding pseudocapsule for at least part of the circumference of the tumor (arrow number 1). It is important that the tumor is inked for margins as complete excision is almost always the goal of the surgeon. In this case the tumor near areas of hemorrhage to the left was not completely excised. Solitary fibrous tumors often feature a central nidus of fibrosis (arrow and number 2). This fibrotic area has implications for interpretation of the MRI scan (see above). Solitary fibrous tumor may show relatively dense cellularity (arrow and number 3) surrounding the central collagenous area. The two photographs, the gross and the very low power are in exactly the same oritentation so you can compare the gross with the histology. At higher magnification the collagenous area is shown (green light arrow centrally). Note the branching vasculature that surrounds the collagenous area. Solitary fibrous tumor may have a somewhat interweaving pattern of tumor among its characteristic branching vasculature. At high magnification from a cellular area numerous mitotic figures (arrows 4) are identified. Some cells are pleomorphic (arrow 5) with bizarre hyperchromatic nuclei. Solitary fibrous tumors present a variable number of mitotic figures as well as pleomorphism. Unfortunately these features do not correlate with clinical aggressiveness (see prognosis below). Immunohistochemistry is characteristic and is included in the original definition of this tumor in the orbit. Tumor cells react with antibodies to CD 34, Bcl2, and CD99. The immunoreactivity to anti-CD 34 (arrow 7) is necessary for the diagnosis. These antibodies are not specific in themselves but when coupled with the characteristic histology, provide a solid diagnosis for present day classification. By the way the immunohistochemistry slide often highlights the branching vasculature (arrow 6).
Treatment: Complete resection is usually recommended for this tumor as the tumor may have an aggressive growth pattern. Radical surgery in other sites such as the pleura improves the eventual outcome.
Prognosis: About 30% of the tumors will pursue an aggressive course with local recurrence and some may metastasize. Interestingly, the histology is a poor predictor of this aggressive course. 1/3 of patients with an aggressive course had histologic signs of malignancy such as a high mitotic index. And almost ½ of the patients with an aggressive clinical course had no signs of histologic malignancy. In the other cases, the presence or absence of signs of malignancy was not published.