SOLITARY FIBROUS TUMOR OF THE ORBIT
Solitary fibrous tumor (SFT), an uncommon but well-recognized typically pleura-based tumor, has only recently been described at other sites. Up to now SFT has been described in numerous extrapleural sites including lung parenchyma, breast, abdominal cavity, mediastinum, skin, parotid gland, meninges, spinal cord and the orbit (1, 2). Twenty-three cases of SFT of the orbit have been reported to date, including 5 cases arising in the lacrimal sac; 11 were in women and 12 in men, with ages ranging from 21 to 75 years. Nonetheless, the actual number of cases is probably higher due to unreported and unrecognized cases (3-10). Gross, histopathological, immunohistochemical, ultrastructural and radiological features of orbital SFT are similar to their counterparts elsewhere.
Patients with orbital SFT usually present with painless, slowly progressive unilateral proptosis or orbital swelling which has developed over months to years (3-10). Imaging studies usually show a well-circumscribed retrobulbar or lacrimal sac mass with relatively homogeneous contrast enhancement. Sometimes there is evidence of smooth remodeling of the orbital bones, but destruction of orbital bones is very uncommon. T2-weighted magnetic resonance imaging (MRI) typically demonstrates a hypointense tumor filling most of the orbit or expanding the lacrimal sac. Intracranial extension is very uncommon (3-10).
Histopathologically, SFTs are well circumscribed or encapsulated and composed of bland spindle cells with some hypercellular areas and myxoid to collagenized backgrounds. Areas with a hemangiopericytomatous vascular pattern and focally increased cellularity are present in some cases. Mitotic activity is usually absent or scanty and necrosis is uncommon. Spindle cells display vimentin immunoreactivity in all and CD34 in almost all cases, and display no cytokeratin, CD31, S-100, epithelium membrane antigen (EMA) or factor VIII immunoreactivity. Fibroblastic differentiation is present ultrastructurally (3-10).
The histopathological differential diagnoses of SFT of the orbit are broad and include benign fibrous histocytoma, HPC, schwannoma, fibrous meningioma and leiomyoma. Benign fibrous histocytomas usually display a characteristic storiform organization of fibroblasts sometimes intermixed with round histocytic-like cells and multinucleated giant cells; and they are usually CD34 negative. HPC contains round to spindle cells around thin-walled, endothelium-lined vascular channels that usually exhibit a staghorn configuration, and individual tumor cells are surrounded by reticulum fibers. Some SFTs can have focal areas of hemangiopericytomatous differentiation, however, CD34 immunoreactivity for SFT tends to be more diffuse and intense than that in HPC (9,11). Although CD34 immunoreactivity can be seen in some schwannomas, characteristic features including alternating areas of Antoni A and Antoni B, Verocay bodies and strong diffuse S-100 immnuostaining usually differentiate schwannoma from SFT. Leimyomas usually show strong immunoreactivity for muscle markers such as desmin and muscle specific actin (11). Fibrous meningiomas also mimic of SFTs. Typically fibrous meningiomas exhibit EMA and cytokeratin immunoreactivity but no CD34, although some cases may display weak patchy CD34 immunrectivity (12). In addition, electron microscopic studies can also be very helpful in the differential diagnosis of SFT.
Similar to SFTs of other anatomic sites, orbital SFTs usually behave in a benign or occasionally, locally aggressive fashion, with as yet no metastatic potential documented. Among all 23 reported cases of orbital SFTs, only one patient developed 2 recurrences 4 and 6 years after initial resection (10). However, long-term prognosis remains uncertain due to the paucity of reports with long-term follow up.
In summary, awareness and recognition of orbital SFT as a distinct pathologic entity should be emphasized due to its ability to mimic others and the existence of occasional aggressive form. In this case, the tumor had a benign histology and surgical excision should be curative. Histologic examination, immunohistochemical analysis and electron microscopy may help in the diagnosis of SFT, and the treatment of choice is en bloc excision.
Contributed by Xuemo Fan, MD, PhD, Terry M Semchyshyn, MD, Louise A Mawn, MD, James B Atkinson, MD, PhD, James C Anderson, MD, Steven A Toms, MD, MPH and Mahlon D Johnson, MD, PhD