Introduction: Primary sinovial sarcoma (PSS) is a very rare malignancy in lung, although lung is the most common organ-based site for metastatic synovial sarcoma. It is a high grade malignant mesenchymal neoplasm showing variable epithelial differentation and characterized by a balanced translocation t(X, 18). Case report: We report a case of a 67- year-old male presented with long lasting cough, chest pain and fever. Chest radiography and computer tomography scan revealed a large well defined lung mass invading hilum of the lung, which was indicated for the lung resection. Histopatological examination showed high-cellulare spindle cell malignant tumor. Immunohistochemically,neoplastic cells were positive for vimentin, TLE-1, Bcl-2, focally positive for cytokeratin AE1/AE3, cytokeratin 7, cytokeratin 18, cytokeratin 19, EMA, calponin and negative for CK 5/6, S100, STAT6, CD34, SMA, Desmin, CD99 and CD 117, which determinate a diagnosis of monophasic PSS of lung. Specific translocation t(X, 18) was detected by fluorescent in situ hybridization (FISH). Conclusion: Diagnosis of PSS in lung may create various diagnostic challenges. It can be established only after exclusion of metastatic sarcoma of other localizations and spindle cell primary lung malignancies. Molecular testing proved to be very helpful or necessary when spindle cell synovial sarcoma was recognized in uncommon sites.

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