Solitary fibrous tumor

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 tumors of the dura
• solitary fibrous tumors of the liver
• solitary fibrous tumor of the orbit
• solitary fibrous tumors of the pleura
• solitary fibrous tumor of the spinal cord

Terminology

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

Epidemiology

Solitary fibrous tumors occur at essentially any age but are uncommon in childhood and somewhat more common in the 5^th and 6^th decades ¹³. 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. 

Clinical presentation

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.

Pathology

Location

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%

Macroscopic appearance

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. 

Microscopic appearance

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. 

Genotype

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. 

Immunohistochemistry

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 malignant solitary fibrous tumors

Radiographic features

CT

CT reveals a well-circumscribed, smooth, and lobulated soft tissue mass that may contain scattered calcifications. Smaller tumors tend to enhance homogeneously, whereas larger lesions may have central tubular or rounded low-attenuation areas due to cystic or necrotic change.

MRI

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

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

However, when a central focus of heterogeneity and variable contrast enhancement is identified in an SFT at CT or MRI, malignant degeneration should be considered ⁷.

Treatment and prognosis

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

In contrast, malignant/higher-grade tumors tend to be larger at the time of diagnosis (>10 cm), more commonly encountered in older individuals and have the expected histological features of malignancy (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.

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

Differential diagnosis

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 synovial sarcomas, malignant peripheral nerve sheath tumors, liposarcoma, etc. ¹³