Solitary fibrous tumours are a rare neoplasm of mesenchymal origin that comprise less than 2% of all soft tissue tumours ranging from indolent tumours 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 tumours, please refer to:
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Terminology
Solitary fibrous tumour is the preferred term replacing haemangiopericytoma in all regions 13.
Epidemiology
The incidence is ~0.6 (meningeal) and ~0.4 (extra-meningeal) per million persons per year 14. Solitary fibrous tumours occur at essentially any age but are uncommon in childhood and somewhat more common in the 5th to 7th decades 13,14. Tumours 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 tumours is usually due to local mass effect and therefore will depend on the location and size of the tumour. Tumours in the head and neck tend, therefore, to present at a smaller size, whereas pleural and abdominal tumours later 13.
Rarely solitary fibrous tumours result in paraneoplastic non-islet cell tumour hypoglycaemia (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
The classification of solitary fibrous tumours avoids the use of "typical" and "malignant" with a proposed risk classification of low aggressive, high aggressive, and dedifferentiated solitary fibrous tumours, which notes that tumour behaviour is along a continuum 14.
Location
Solitary fibrous tumours 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 tumours appear as sharply marginated multilobulated firm masses. The cut surface is dense and pale in colour although areas of fat, haemorrhage, necrosis and myxomatous change can be encountered especially in higher-grade tumours. These more aggressive tumours may also demonstrate less well-defined margins with infiltration into adjacent tissues 13.
Microscopic appearance
Solitary fibrous tumours have variable cellularity but generally are composed of a collagenous matrix with arrays of spindle cells. Areas of necrosis, cystic or myxoid change, calcification, haemorrhage, increased vascularity, atypia, or malignancy may also be seen 7,8. Blood vessels often contain large calibre thin-walled branching staghorn blood vessels 13.
More cellular tumours tend to represent higher grade lesions 13.
Immunophenotype
Immunohistochemical analysis can play an important role in the diagnosis and management. Solitary fibrous tumours 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 tumours (~75% positive)
BCL-2 (B-cell lymphoma 2 protein): a marker of terminal differentiation
cytokeratin, S-100, and p53 proteins have shown increased expression in aggressive solitary fibrous tumours
Genetics
Solitary fibrous tumours, regardless of location, are characterised by the genomic inversion of 12q13 locus resulting in the fusion of NAB2 and STAT6 genes 13.
Radiographic features
CT
CT reveals a well-circumscribed, smooth, and lobulated soft tissue mass that may contain scattered calcifications and are high density 14. These tumours enhance avidly, with smaller tumours 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 tumours 14.
MRI
On MRI, benign solitary fibrous tumours 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 haemorrhage 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 tumour.
However, when a central focus of heterogeneity and variable contrast enhancement is identified in a solitary fibrous tumour on CT or MRI, higher risk tumours should be considered 7.
Treatment and prognosis
Lower risk solitary fibrous tumours tend to be smaller (<10 cm) in size and occur in younger individuals 14. They typically have a favourable outcome with surgical resection alone.
In contrast, high risk tumours 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.
History and etymology
In 1870, Wagner described solitary fibrous tumours for the first time 12,13. Klemperer and Rabin did the first morphological characterisation of solitary fibrous tumours in 1931 12,13.
Differential diagnosis
Solitary fibrous tumours 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 tumours, liposarcoma, etc. 13
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