Solitary Fibrous Tumors

Excerpt

The historical evolution of solitary fibrous tumors (SFTs) dates to the late nineteenth century, when Wagner first described pleural lesions resembling benign mesothelioma in 1870. Klemperer and Rabin clearly delineated the entity in 1931, who reported 5 cases of primary pleural neoplasms arising from submesothelial areolar tissue rather than the mesothelial lining. In 1942, Stout and Murray introduced the term hemangiopericytoma (HPC) to describe a morphologically similar vascular spindle-cell tumor of pericytic origin. In 1951, Stout and Himadi described a series of 8 well-circumscribed pleural tumors, then referred to as solitary (localized) mesothelioma, that are now recognized as SFTs.

Initially regarded as separate entities, subsequent morphologic, immunohistochemical, and molecular studies demonstrated that SFTs and HPCs represent morphologic variants within a single fibroblastic spectrum unified by the NAB2-STAT6 gene fusion. This concept, first articulated by Gengler and Guillou, is now reflected in the World Health Organization (WHO) classification, which consolidates SFTs and HPCs into a single clinicopathologic spectrum. Recognition of extrapleural SFTs expanded after the first large series in 1991, establishing that these tumors can arise in virtually any anatomic site, including the meninges, retroperitoneum, abdomen, and extremities.

SFTs are rare fibroblastic mesenchymal neoplasms, accounting for approximately 3.7% of visceral and soft-tissue sarcomas. They represent a heterogeneous group of fibroblastic mesenchymal tumors that are typically benign but may rarely exhibit malignant behavior with metastatic potential. Although most follow an indolent course, a subset demonstrates aggressive behavior with local recurrence or distant metastasis. They can arise in a wide range of anatomic sites and typically present as well-circumscribed, slow-growing masses that remain asymptomatic until large enough to compress adjacent structures.

Diagnosis can be challenging because of variable histologic patterns and overlap with other spindle-cell tumors. The discovery of the NAB2-STAT6 gene fusion, and its surrogate marker nuclear STAT6 immunostaining, has markedly improved diagnostic precision. Nevertheless, distinguishing SFTs from morphologic mimics remains critical because management and prognosis differ significantly across the spectrum of fibroblastic neoplasms.