Cytologic and Histologic Findings of Extrapleural Solitary Fibrous Tumor: Report of Two Cases.

Solitary fibrous tumors (SFT) are a rare neoplasm of mesenchymal origin. SFT was previously described primarily in the pleura and meninges; however, extrapleural and extra-meningeal SFT have been reported in almost every anatomic site and account for up to 40% of cases. The most significant histologic findings of SFT include spindle cell proliferation in a "pattern-less pattern", dilated and branching "staghorn"-like vasculature, and ropey collagen deposition. However, these findings are not consistently present in every case of SFT and may also be seen in other diseases. SFT has a characteristic NAB2::STAT6 gene fusion and nuclear overexpression of STAT6. The rarity of the disease, broad range of differential diagnoses, and wide spectrum of cytomorphological and histologic findings make the diagnosis of extrapleural SFT, especially on a fine needle aspiration (FNA) specimen, challenging. Recognizing and including this entity in the differential is necessary before the final diagnosis may be achieved through proper immunohistochemical and molecular workup. In this paper, we present two cases of extrapleural SFT with unusual locations: the first is a primary SFT present in a parotid gland and the second is a metastatic SFT present as two solid pancreatic masses.