Solitary fibrous tumor

Last revised by Mohammad Taghi Niknejad on 12 Jan 2024

Solitary fibrous tumors are a rare neoplasm of mesenchymal origin that comprise less than 2% of all soft tissue tumors ranging from indolent tumors to more aggressive masses.

They can be very large and can occur essentially anywhere, although some areas are more characteristic than others. For a discussion of specific presentations of solitary fibrous tumors, please refer to:

Solitary fibrous tumor is the preferred term replacing hemangiopericytoma in all regions 13

The incidence is ~0.6 (meningeal) and ~0.4 (extra-meningeal) per million persons per year 14. Solitary fibrous tumors occur at essentially any age but are uncommon in childhood and somewhat more common in the 5th to 7th decades 13,14. Tumors of the pleura are reported in slightly older age groups 13 although this may be due to a combination of delayed presentation and the heterogeneity of published series. 

The clinical presentation of solitary fibrous tumors is usually due to local mass effect and therefore will depend on the location and size of the tumor. Tumors in the head and neck tend, therefore, to present at a smaller size, whereas pleural and abdominal tumors later 13

Rarely solitary fibrous tumors result in paraneoplastic non-islet cell tumor hypoglycemia (NICTH) known as Doege-Potter syndrome which results from the secretion of insulin-like growth factor-2 (IGF-2) 4-7,11,13.

Rarer still is acromegaly 11.

The classification of solitary fibrous tumors avoids the use of "typical" and "malignant" with a proposed risk classification of low aggressive, high aggressive, and dedifferentiated solitary fibrous tumors, which notes that tumor behavior is along a continuum 14.

Solitary fibrous tumors occur essentially anywhere, both within lined cavities (e.g. pleura, peritoneum, dura), solid organs (e.g. liver, spleen) and soft tissues (e.g. extremities, orbit, retroperitoneum). Some locations are more common than others 7,13:

  • pleura (most common): ~30%

  • meninges (intracranial and spinal cord 2-3): ~25%

  • abdominal cavity and organs: ~20%

  • trunk: ~10%

  • extremities: ~10%

  • head and neck: ~5%

Solitary fibrous tumors appear as sharply marginated multilobulated firm masses. The cut surface is dense and pale in color although areas of fat, hemorrhage, necrosis and myxomatous change can be encountered especially in higher-grade tumors. These more aggressive tumors may also demonstrate less well-defined margins with infiltration into adjacent tissues 13

Solitary fibrous tumors have variable cellularity but generally are composed of a collagenous matrix with arrays of spindle cells. Areas of necrosis, cystic or myxoid change, calcification, hemorrhage, increased vascularity, atypia, or malignancy may also be seen 7,8. Blood vessels often contain large caliber thin-walled branching staghorn blood vessels 13

More cellular tumors tend to represent higher grade lesions 13

Immunohistochemical analysis can play an important role in the diagnosis and management. Solitary fibrous tumors are usually positive for the following markers 12,13:

  • STAT6 nuclear staining as a surrogate marker of NAB2-STAT6 gene fusion (most sensitive and specific)

  • CD34: a marker for normal endothelium and vascular tumors (~75% positive)

  • CD99

  • BCL-2 (B-cell lymphoma 2 protein): a marker of terminal differentiation

  • vimentin

  • cytokeratin, S-100, and p53 proteins have shown increased expression in aggressive solitary fibrous tumors

Solitary fibrous tumors, regardless of location, are characterized by the genomic inversion of 12q13 locus resulting in the fusion of NAB2 and STAT6 genes 13

CT reveals a well-circumscribed, smooth, and lobulated soft tissue mass that may contain scattered calcifications and are high density 14. These tumors enhance avidly, with smaller tumors homogeneous whereas larger lesions may have central tubular or rounded low-attenuation areas due to cystic/necrotic change 14. Large collateral vessels are present in around one-third of tumors 14.

On MRI, benign solitary fibrous tumors usually have relatively homogeneous low-to-intermediate T1 and T2 signal intensity relative to skeletal muscle 14 because of fibrous tissue, as well as intense enhancement.

In addition, there may be areas of subacute hemorrhage that have high T1 signal intensity, as well as non-enhancing cystic or necrotic foci that are more heterogeneous and higher in T2 signal intensity relative to the remainder of the tumor.

However, when a central focus of heterogeneity and variable contrast enhancement is identified in a solitary fibrous tumor on CT or MRI, higher risk tumors should be considered 7.

Lower risk solitary fibrous tumors tend to be smaller (<10 cm) in size and occur in younger individuals 14. They typically have a favorable outcome with surgical resection alone.

In contrast, high risk tumors tend to be larger at the time of diagnosis (>10 cm), more commonly encountered in older individuals and have the expected histological features (increased cellularity, increased mitotic activity, nuclear pleomorphism, etc.) as well as necrosis and local infiltration 13. These have a poorer prognosis, especially if incompletely resected.

Metastasis rates can be as high as ~35% (range 25-45%) with late recurrence common (up to 10%) 14.

Optimal adjuvant therapy for this group is unknown, and close-interval follow-up is advised because there is an increased incidence of local recurrence.

In 1870, Wagner described solitary fibrous tumors for the first time 12,13. Klemperer and Rabin did the first morphological characterization of solitary fibrous tumors in 1931 12,13.

Solitary fibrous tumors have a wide differential that largely depends on their location. For example, intracranial they very closely mimic meningiomas

Elsewhere, other sarcomas can appear very similar, including angiosarcoma, synovial sarcomas, malignant peripheral nerve sheath tumors, liposarcoma, etc. 13

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