Fibrous histiocytoma and fibrous tissue tumors of the orbit.
Fibrous orbital tumors present clinically and radiographically in a broad spectrum ranging from a benign mass, to locally aggressive tumor, to invasive malignancy. Pathologic analysis and diagnosis are often challenging, usually based on a combination of light microscopy, immunohistochemistry, and electron microscopic findings. Some light microscopic and immunohistochemical findings, however, are relatively characteristic. A storiform or cartwheel pattern and vimentin staining are characteristic of fibrous histiocytoma. A herringbone pattern is usually found in fibrosarcoma. A "patternless pattern" and CD34 staining is found most commonly in solitary fibrous tumor. CT and MR imaging findings, as well as clinical presentation, in fibrous orbital lesions are often difficult to distinguish from those of other orbital masses, although there may be useful clues. Benign fibrous lesions are usually well-circumscribed and may chronically remodel bone, whereas more aggressive malignant fibrous tumors tend to have infiltrating margins and may destroy bone on CT or MR imaging. With malignant fibrous masses, enhancement patterns on CT or MR imaging may be more inhomogeneous, with avascular or necrotic nonenhancing regions. At MR imaging, benign lesions tend to be homogeneous on T1, T2, and postgadolinium T1-weighted images, whereas malignant soft tissue lesions may change their pattern from homogeneous on T1-weighted images to heterogeneous with low signal septations on T2-weighted images. Low T2 signal comprising part or all of a fibrous lesion correlates with dense collagen fibers, with a less cellular matrix. Areas of hyperintensity on T2-weighted images correspond with a more cellular matrix of fibroblasts and other cells. Calcification within a tumor, however, may give a similar appearance. Thus, if a lesion has predominantly low signal on T2-weighted images, or less specifically has low signal septations, then a fibrous orbital lesion with high collagen content may be ranked higher in the differential diagnosis (see Figs. 2E and 3B). When T2 signal is intermediate-to-high, then one has a difficult time narrowing the differential diagnosis. Radiographically, distinguishing these lesions from other fibrous orbital lesions, as well as from other varieties of orbital masses, is difficult. Differential diagnosis of fibrous orbital masses includes all the fibrous lesions described in this article, in addition to schwannoma (Fig. 7), neurofibroma (Figs. 4 and 8), hemangiopericytoma (Figs. 9 and 10), rhabdomyosarcoma, meningioma, lymphoma, and metastasis (Figs. 11 and 12). A history of prior orbital irradiation for retinoblastoma or other tumors may raise the possibility of radiation-induced secondary tumors, such as MFH, fibrosarcoma, and osteosarcoma. Determining the extent of orbital involvement remains the primary goal of the radiologist. The final diagnosis still rests with the pathologist.