Solitary fibrous tumor of the orbit

Last revised by Daniel J Bell on 23 Sep 2021

The solitary fibrous tumor of the orbit is a rare spindle-cell neoplasm originating from mesenchymal fibroblast-like cells histologically identical to solitary fibrous tumors found elsewhere

Solitary fibrous tumors occur in a wide age range reported from 9 to 76 years without a convincing gender predilection, although some reports have found increased incidence in females with the male-to-female ratio of 1:2

The patients usually present with slowly progressive painless unilateral exophthalmos or swelling or palpable mass in the periocular area, 

On microscopic examination, solitary fibrous tumors are well-circumscribed, non-encapsulated tumors that show a pattern-less arrangement of alternating hypercellular and hypocellular regions of spindle cells against a collagenous background of variable vascularity. A strongly positive reaction for immune-histochemical marker CD34 is now believed to be the most important marker for diagnosis.

Solitary fibrous tumors are also immune-reactive for vimentin and B-cell lymphoma 2 (bcl-2) but negative for keratin, cytokeratin, epithelial membrane antigen, S-100 protein, smooth muscle actin, factor VIII-related antigen, and Desmin. 

Solitary fibrous tumor usually present as a solitary well-circumscribed ovoid mass located in intraconal and extraconal spaces of the orbit, lacrimal gland, lacrimal sac, and eyelid.

Solitary fibrous tumors are well-circumscribed and iso-dense to hyperdense compared to muscle / cerebral cortex. Occasional internal calcifications and necrosis have also been reported. Although remodeling of the adjacent bones may be seen in large, long-standing lesions, frank bone destruction is an exceptional finding that should prompt suspicion for a malignant tumor.

The lesions show marked homogeneous or heterogeneous enhancement on post-contrast images very similar enhancing characteristics to those of the internal carotid artery which is attributed to high vascularity because of the prominent vascular channels within the tumor. The lesions demonstrate rapid enhancement and early washout of contrast.

These tumors exhibit the following signal characteristics:

  • T1: homogeneous isointense signal intensity typically similar to cerebral cortex or muscle
  • T2:
    • iso- to hypointensity (the best clue to the diagnosis)
    • heterogeneous mixed iso to hypointensity, reflecting fibrous tissue with high collagen content
    • areas of intralesional T2 hyperintensity related to internal hemorrhage, cystic degeneration, or relatively fresh fibrosis
  • T1 C+: diffuse heterogeneous vivid contrast enhancement seen on post-contrast images

Although local excision is usually curative, orbital solitary fibrous tumors have been reported to recur and may rarely metastasize. Local recurrences are usually related to incomplete excision. Although extremely rare, malignant degeneration has also been reported. 

The differential diagnosis is that of a highly vascular orbital lesion and includes

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