Usefulness of SS18-SSX antibody as a diagnostic marker for pulmonary metastatic synovial sarcoma

BACKGROUND: The novel SS18-SSX fusion-specific antibody is reported to have high sensitivity and specificity for the diagnosis of primary synovial sarcoma (SS), which often metastasizes to the lung. Thus far, no study has validated the diagnostic efficacy of SS18-SSX antibody for pulmonary metastatic SS. Therefore, we aimed to investigate the usefulness of the SS18-SSX antibody in the diagnosis of pulmonary metastatic SS. METHODS: We evaluated the immunohistochemistry of SS18-SSX fusion-specific antibody (E9X9V) in 10 pulmonary metastatic SS cases and the corresponding five primary sites (four limbs and one mediastinum) in five patients, for whom SS was already diagnosed and confirmed by fluorescence in-situ hybridization in the metastatic and primary sites, and in 93 clinical and histologic mimics including 49 non-SS, pulmonary metastatic sarcomas, 39 primary lung cancers, and five intrathoracic solitary fibrotic tumors. All specimens were surgically resected at Shinshu University Hospital during 2001–2019. For primary and metastatic SS, we also evaluated SS18-SSX immunohistochemistry using needle biopsy and touch imprint cytology specimens from the primary site. RESULTS: SS18-SSX staining was diffusely-strongly positive in all 10 pulmonary metastatic SS cases and the corresponding five primary sites; whereas, it was negative in all 93 clinical and histologic mimics (100% sensitivity and 100% specificity). Further, SS18-SSX staining was also sufficiently positive in the biopsy and cytology specimens. CONCLUSIONS: Immunohistochemistry of the SS18-SSX fusion-specific antibody is useful for the differential diagnosis of pulmonary metastatic SS in clinical practice. This simple and reliable method has the potential to replace traditional genomic tests. However, further studies are warranted in this regard.