Meningeal Solitary Fibrous Tumor: A Cytological Report With Emphasis on the Usefulness of Immunocytochemical Analysis for <scp>STAT6</scp>

Solitary fibrous tumors (SFTs) are rare neoplasms characterized by spindle neoplastic cell proliferation within collagenous stroma and prominent dilated vasculature. They present a wide histopathological spectrum, ranging from hypocellular lesions with a rich collagenous stroma to hypercellular lesions with scant stroma. Meningeal SFTs are usually hypercellular, and their cytological features remain poorly characterized. This brief report presents a cytological description of a meningeal SFT with immunocytochemical analysis for signal transducer and activator of transcription 6 (STAT6). A 55-year-old Japanese male, with a history of surgical resection of SFT 11 years earlier, was discovered to have a recurrent tumor in the left cerebellar tentorium. An intraoperative touch smear of the tumor revealed loosely aggregated hypercellular clusters and scattered round-to-oval spindle cells in a clear background, with large nuclei and hyperchromasia. Although minimal stromal components were observed, no dilated vascular structures were identified. Immunocytochemistry showed diffuse positive immunoreactivity for STAT6, confirming the diagnosis of meningeal SFT. Histopathological and immunohistochemical analyses confirmed recurrent meningeal SFT. The cytological features of meningeal SFT differ from those of pleural SFT in terms of minimal collagenous stroma and the presence of nuclear atypia. STAT6 is a specific marker for SFT, and immunocytochemical staining is useful for its diagnosis.