Solitary fibrous tumor of the thorax

Solitary fibrous tumors of the thorax are rare pleural-based tumors that account for <5% of all primary pleural tumors ¹⁶. They are of variable behavior but are mostly indolent, benign-behaving tumors although can be aggressive and can rarely transform into undifferentiated sarcoma.

Terminology

Throughout much of the 20^th century, there was a continual debate about the precise histology of both normal pleura and pleural tumors. The properties of the mesothelial cell itself were controversial, mostly due to its unique expression of both mesenchymal and epithelial attributes.

Gradually, a distinction was recognized between "localized" versus "aggressive" mesothelial tumors (i.e. "malignant" mesothelioma). Solitary fibrous tumors have been variably termed benign mesothelioma, localized fibrous tumor of the pleura, localized mesothelioma, localized fibrous mesothelioma, localized benign fibroma, pleural fibroma, submesothelial fibroma, hemangiopericytoma, or giant cell angiofibroma ^10,16. "Solitary fibrous tumor of the thorax" is now the recommended term in the WHO classification of thoracic tumors (5^th edition) ¹⁶.

Epidemiology

Solitary fibrous tumors usually present in the 6^th to 7^th decades with no gender predilection ¹⁶. They have an incidence of 2.8 in 100,000 ¹¹.

Associations

• hypoglycemia (2-4%) ⁶: thought to be due to the production of insulin-like growth factor 2 (known as Doege-Potter syndrome)

• hypertrophic pulmonary osteoarthropathy (~20%) ⁶: thought to be due to abnormal production of hyaluronic acid

• asbestos exposure is not an association ¹⁵

Diagnosis

The diagnostic criteria according to the WHO classification of thoracic tumors (5^th edition) are ¹⁶:

• essential: well-circumscribed mass; spindled to ovoid cells arranged around a branching and hyalinized vasculature; variable stromal collagen deposition; CD34 and/or STAT6 expression by immunohistochemistry

• desirable (in selected cases): demonstration of NAB2-STAT6 gene fusion

Clinical presentation

Usually asymptomatic and discovered as an incidental finding ³. Of those who are symptomatic, the clinical presentation can be with either a cough, chest pain, or shortness of breath ¹⁶. 

Pathology

The etiology of solitary fibrous tumors is unknown. They are mostly slow-growing and benign-behaving, although a significant minority will demonstrate aggressive behavior ¹⁶. Rarely, they can transform into undifferentiated sacroma ¹⁶.

Location

There may be a predilection towards the mid to lower zones of the chest. They arise from visceral pleura (~80%), parietal pleura, lung parenchyma (3%), pericardium, or rarely the mediastinum ^12,16.

Microscopic appearance

Solitary fibrous tumors consist of uniform fibroblastic spindle cells with varying cellularity, featureless architecture, fibrosis and branching staghorn vessels. Myxoid or cystic degeneration can occur ¹⁶.

Classification

The use of "benign" or "malignant" in solitary fibrous tumors is not recommended. Risk stratification for likelihood of aggressive behavior (into low, intermediate, high categories) is preferred over staging ¹⁶. Various risk models have been proposed ¹⁶.

Radiographic features

Plain radiograph

Presents as a pleural-based mass, which tends to be relatively circumscribed, but can also be lobulated. It often forms an obtuse angle with the chest wall and may grow to a large size. Pedunculated lesions can change position and appearance with respiration or with a change in position on serial radiographs. 

Calcification, rib destruction, and pleural effusions are typically not associated features.

CT

Tends to have soft tissue attenuation on unenhanced scans and shows relatively homogeneous intense background enhancement on contrast-enhanced scans (from rich vascularization). Non-enhancing areas may be present, corresponding to necrosis, myxoid degeneration, or hemorrhage within the tumor. A pedicular attachment may also be seen. The incomplete border sign may be present in the portion of the mass contiguous with the pleura.

MRI

Due to the fibrous component, signal characteristics tend to be:

• T1: typically low to intermediate signal

• T2: typically low signal overall (thought to be due to high cellularity and abundant collagen); areas of necrosis and myxoid degeneration can have high signal

• T1 C+ (Gd): In general, increased vascularity causes intense enhancement post intravenous gadolinium administration ¹⁴

MRI may also show necrotic, hemorrhagic, and cystic components in greater detail if these are present.

Treatment and prognosis

The majority of tumors tend to be indolent and slow-growing. Aggressive tumors occur at a rate of ~30% (range 10-57%) ¹². Surgical resection is the treatment of choice ^5,6. Chemotherapy and radiotherapy are used in the minority (<5%) of patients ¹². 

Solitary fibrous tumors can metastasize (in order of frequency) to the pleura, chest/abdominal wall, lung, peritoneum, and/or liver. Metastases to the brain or bone are rare ¹³. 

Recurrence rates are ~18% (range 10-25%) and are higher with incomplete resection and a higher risk category ¹⁶. The 5-year survival is ~85% ¹². 

History and etymology

Understanding of primary pleural neoplasia has changed drastically since the late 1800s, and so the associated terminology has also changed. German pathologist Ernst Leberecht Wagner (1829-1888) is generally credited with the first description of pleural mesothelioma/fibroma in 1870, although he described it "tuberkelähnliche" lymphadenome ("tubercle-like" lymphadenoma) ⁹. Primary pleural malignancies were rare until the 20^th century when the incidence increased considerably due to environmental exposures.

Differential diagnosis

Considerations for extremely well-defined lesions include:

• pleural lipoma

• intercostal nerve neurilemoma (schwannoma)

If not extremely well defined, consider:

• organized inflammation 

• peripheral lung cancer ¹²

• localized mesothelioma ¹²

• solitary pleural metastasis

Also, consider the differential for a single pleural mass.