Pathology of Solitary Fibrous Tumour 

Dr Sampurna Roy MD              2022

Extrapleural solitary fibrous tumour is a rare mesenchymal tumour characterized by prominent hemangiopericytoma-like branching pattern.

Because of the occasional aggressive behavior of this tumour, it should be considered potentially malignant.

The recurrence rate is very low after surgical resection, with a slight increased risk of recurrence in the extrathoracic tumours.

Age:  

Usually occurs in middle aged adults between 20 and 70 years. The lesion may rarely occur in adults.

Sites: 

This tumour has been reported in extrapleural sites, including the orbit, liver, salivary glands, tongue, nose, paranasal sinuses, larynx, thoracic wall, mediastinum, retroperitoneum,  thyroid , gastrointestinal tract, hypoglossal nerve & breast.

Other locations include adrenal, urinary bladder, prostate, spermatic cord, testes, meninges, spinal cord and periosteum.

Clinical presentation:

This is a well circumscribed slow growing painless tumour.

Clinical symptoms depend on the size and location of the tumor, where symptoms are usually related to compression rather than infiltration of the adjacent tissue.

Solitary fibrous tumour most often manifests as a benign lesion; however, malignant transformation and metastasis have been reported in a number of both pleural and extrapleural tumours.

Gross features: 

Well circumscribed, lobulated, partially encapsulated tumour.

The size of the tumour ranges from 1 to 25 cms in diameter.

The tumour has a tan white, firm, homogenous cut surface.

There may be focal haemorrhagic and myxoid areas.

Microscopic features: 

The solitary fibrous tumors are composed of relatively uniform proliferating ovoid or spindle-shaped cells that are arranged haphazardly, often in a fascicular and whorled pattern, in a heavily collagenized stroma.

Often there are abundant blood vessels arranged in a hemangiopericytoma-like pattern .

The degree of cellularity varies from area to area and in less cellular areas the intercellular collagen is often thick and keloid-like. 

Some tumours show compact clusters of polygonal cells with a rather epithelioid appearance.

Mitotic activity is generally low, ranging from 0 to 2/10 HPFs. Focal areas of necrosis are identified in some cases.

Stromal cystic degeneration and prominent myxoid change (Myxoid Solitary Fibrous Tumour) are also present in some cases.

Prognostic factors for a malignant potential include high cellularity,mitotic activity (>4 per 10 high-power fields), pleomorphism, extensive hemorrhage, and necrosis.

Also, tumours larger than 10 cm should be closely monitored.

Rarely the tumour may contain mature adipose tissue- Lipomatous Hemangiopericytoma. :  

1. Lipomatous hemangiopericytoma: a fat-containing variant of solitary fibrous tumor? Clinicopathologic, immunohistochemical, and ultrastructural analysis of a series in favor of a unifying concept.

2. Lipomatous hemangiopericytoma. A histologic, ultrastructural and immunohistochemical study of a unique variant of hemangiopericytoma.

3. Lipomatous hemangiopericytoma: a morphologically distinct soft tissue tumor.

4. Lipomatous hemangiopericytoma: a rare variant of hemangiopericytoma that may be confused with liposarcoma.

Giant cell angiofibroma may be a giant-cell rich variant of solitary fibrous tumour. Orbital and extraorbital giant cell angiofibroma: a giant cell-rich variant of solitary fibrous tumor? Clinicopathologic and immunohistochemical analysis of a series in favor of a unifying concept.

Immunohistochemistry: 

Immunohistochemically there is characteristic expression of  CD34, bcl-2, and CD99.

Factor XIIIa and alpha-smooth muscle actin are less commonly reactive and a very few cells may be faintly positive for factor VIII-related antigen and Ulex europaeus agglutinin 1. 

All cases are essentially negative for S-100 protein, desmin, CD31, and CD68.

Differential diagnosis:   

Solitary fibrous tumour may show a variety of growth patterns which may include fascicular areas, storiform and herringbone formations and wavy neurofibroma-like and monophasic synovial sarcoma-like areas in the solid spindle cell component, thus simulating a variety of soft tissue neoplasms.

The principal differential diagnoses of solitary fibrous tumor are monophasic fibrous synovial sarcomas and nerve sheath tumours.

A variety of other entities, such as leiomyosarcoma, fibromatosis, dermatofibrosarcoma protuberans, hemangiopericytoma, fibrosarcoma are frequently considered in the differential diagnosis of solitary fibrous tumour.

Myxoid solitary fibrous tumour:   

Differential diagnsosis:  Low-grade fibromyxoid sarcoma ; myxoid synovial sarcoma ; malignant peripheral nerve sheath tumour ; low-grade myxofibrosarcoma ; myxoid liposarcoma ; myxoid spindle cell lipoma ;  myxoid neurofibroma, and so-called  "hemangiopericytoma."