Cytological characteristics of meningeal solitary fibrous tumor metastatic to the lung: A case report with immunocytochemical analysis

Solitary fibrous tumor (SFT) is a relatively uncommon mesenchymal tumor, and its occurrence in the meninges is rare. We herein report what is, to the best of our knowledge, the first cytological case of meningeal SFT metastatic to the lung with immunocytochemical analysis for signal transducer and activator of transcription 6 (STAT6), and compare the cytological characteristics to those of pleuropulmonary SFT. A 58-year-old Japanese male patient was found to have multiple nodules in the bilateral lungs after surgery for meningeal SFT. Partial resection of the nodules was performed, and touch smears were obtained. The Papanicolaou smear revealed cohesive hypercellular clusters of polygonal to elongated neoplastic cells with scant cytoplasm and oval to short spindle-shaped nuclei with nucleoli containing coarse chromatin. Mild-to-moderate nuclear pleomorphism was observed. No collagenous stroma was noted. Immunocytochemical analysis revealed that the neoplastic cells diffusely expressed STAT6. Histopathological and immunohistochemical studies confirmed the diagnosis of meningeal SFT metastatic to the lung. Moreover, reverse transcription-polymerase chain reaction analysis revealed that the lung tumor harbored NAB2ex6-STAT6ex16 fusion. Recent studies demonstrated that there is a clinicopathological difference among NAB2-STAT6 fusion variants in SFT. As reflected in fusion variants, meningeal SFT occasionally lacks collagenous stroma, as in the present case, although the most characteristic cytological feature of SFT is the presence of spindle-shaped neoplastic cells embedded in dense collagenous stroma. Thus, immunocytochemical analysis for STAT6 is very useful for diagnosing SFT.