Cystic synovial sarcoma of the pleura mimicking a cystic thymoma: a case report illustrating the role of decreased INI-1 expression in differential diagnosis.

Less than 40 cases of primary pleural synovial sarcoma (SS) have been reported to date. Furthermore, only three cases of cystic SS have been documented in the English literature, including cases originating from sites other than the pleura. Herein, we present an exceedingly rare case of cystic SS originating from the mediastinal side of the visceral pleura in an asymptomatic 47-year-old man, which was detected during a checkup. On contrast-enhanced computed tomography, distinguishing between cystic SS and cystic thymoma was difficult because the tumor was attached to the anterior mediastinum where the latter type of malignancy is more often detected. Histopathological examination showed tumor cells with spindled morphology showing hypercellularity and moderate nuclear atypia, with less than one mitotic figure per high-power field. As these features are associated with both monophasic fibrous SS and type A thymoma, more data was required to determine proper diagnosis, and therefore, immunohistochemistry was performed. Along with a conventional panel of markers, the SS-specific marker integrase interactor 1 (INI-1) was applied and found to be decreased; decreased expression of INI-1 is characteristic of SS. A diagnosis of SS was confirmed by detection of the SYT-SSX fusion gene via fluorescence in situ hybridization. Given the relatively common availability of INI-1 testing in departments of pathology, this protein would be helpful incorporated into the standard panel of markers for diagnosing SS.