Tumors of the pleura are uncommon, and when they arise, are almost always malignant. Malignant mesothelioma and malignant pleural metastases are far more common than benign pleural tumors. Primary benign tumors of the pleura represent only 5% of all pleural tumors and include solitary fibrous tumors, lipomas and lipoblastomas, adenomatoid tumors, and calcifying fibrous tumors. Of these, solitary fibrous tumors are the most common, and will be the primary focus of this chapter. There have been fewer than a thousand cases of solitary fibrous tumor reported in the published literature since the entity was first described by Klemperer and Rabin in 1931.¹

Solitary fibrous tumor has been known for many decades, but its nomenclature has evolved. In the original description in 1931, Klemperer and Rabin classified mesothelioma as either “localized” or “diffuse.”¹ Since that time, solitary fibrous tumor has been previously described as “localized mesothelioma,” “benign mesothelioma,” “solitary fibrous mesothelioma,” or “pleural fibroma.” These terms reflected the incorrect assumption that they were related to malignant mesothelioma of the pleura. More recent research has shown that solitary fibrous tumors originate from mesenchymal cells in the subserosal or submesothelial layer of the pleura, not in the mesothelial layer, which is distinct from mesothelioma (Fig. 129-1).² Moreover, solitary fibrous tumors are not associated with asbestos exposure, further indicating a unique pathophysiologic process in the origin of these tumors.

Solitary fibrous tumors are rare. A contemporary estimate of an age-adjusted incidence indicates a rate of 0.14 per 100,000³ in comparison to mesothelioma, which has an incidence of approximately 2 per 100,000.⁴ Solitary fibrous tumors occur equally in men and women, and its incidence is highest during the fourth to seventh decades of life. It is asymptomatic in approximately half of the cases. In the other half of patients, solitary fibrous tumors can be associated with either intrathoracic or extrathoracic manifestations. Intrathoracic symptoms are related to the mass effect of the tumor itself on the lungs or chest wall, which can result in dyspnea, chest pain, or a chronic cough. Extrathoracic manifestations are paraneoplastic, and manifest as hypertrophic pulmonary osteoarthropathy or clubbing of the digits in 20% (Pierre–Marie–Bamberg syndrome), hypoglycemia in 5% (Doege–Potter syndrome), gynecomastia, or galactorrhea. Constitutional symptoms may also be present, such as fever, fatigue, and weight loss.

Solitary fibrous tumors most commonly arise on the visceral side of the pleura and are usually ovoid in shape (Figs. 129-2 and 129-3). As the name suggests, these tumors are usually isolated. The tumors usually grow in a pedunculated manner, and the pedicle can be highly vascularized. In addition, large aberrant vessels feeding into the tumor have been described. Due to the pedunculated feature, movement of the mass on x-ray with changing of the patient’s position is possible.⁵ Invasion of the lung is uncommon, and such tumors are described as an “inverted” solitary fibrous tumor since the bulk of the tumor is intraparenchymal in the lung.

The differential diagnosis of solitary fibrous tumor includes mesothelioma, sarcomas (synovial sarcoma, fibrosarcoma, nerve sheath sarcoma, malignant fibrous histiocytoma), and hemangiopericytoma. The histology of solitary fibrous tumors reveals their mesenchymal origin, and typically consists of a classic “patternless pattern” of whorled, dense fibrous tissue.⁶ There are spindle cells mixed with collagen and elastin bundles. Cystic structures can be found within the tumor, as can calcification. Immunohistochemistry can be useful to distinguish them from mesothelioma, as routine histology may not give a definitive diagnosis. Staining characteristics of solitary fibrous tumor include positive staining for vimentin, CD34, and bcl-2, while being negative for keratin.⁷