[Clinicopathological features of primary thoracic synovial sarcoma: a study of 42 cases].

<b>Objective:</b> To investigate the clinical and pathological characteristics of primary thoracic synovial sarcoma (PTSS). <b>Methods:</b> Forty-two PTSS cases diagnosed at the Department of Pathology, the First Affiliated Hospital of Zhengzhou University, Zhengzhou, China from October 2011 to April 2024 were analyzed. All cases were retrospectively studied using hematoxylin-eosin staining and immunohistochemistry. Their clinicopathological features were also reviewed. SS18 rearrangement was assessed in 28 cases using fluorescence in situ hybridization (FISH). Next generation sequencing (NGS) was performed on 8 cases. <b>Results:</b> Among the 42 cases, there were 23 biopsies and 19 surgically-removed specimens. One case was a specimen resected after neoadjuvant chemotherapy. There were 22 males and 20 females, with an age ranging from 6 to 68 years. Twenty-nine cases occured in the lung, 6 in mediastinum, 4 in pericardium, 1 in visceral pleura, and 1 in right atrium. One case did not show any unequivocal primary site. Computed tomography showed the tumors were manifested as a cystic mass, a solid mass, or thickening of the pleura and pericardium. Thirty-two cases had respiratory symptoms, while 19 had pleural effusion. One case had a history of radiotherapy for papillary thyroid carcinoma. Nineteen patients were treated with surgery, while 19 were treated with chemotherapy without surgery. Four patients were diagnosed and discharged, without specific treatment on the record. Morphologically, 1 case was biphasic type, 39 cases were monophasic type, and 2 cases were poorly differentiated type. In addition to the typical morphology of synovial sarcoma, tumors also showed pulmonary bullous changes, stromal collagen hyalinization, hemangiopericytoma-like vasculature, stromal edematous myxoid changes, and microcystic structure. Immunohistochemically, all cases were diffusely positive for TRPS1 (22/22), TLE1 (21/22), CD99 (26/26), SS18-SSX (25/25) and INI1 (12/12), including 3 cases with decreased expression of INI1. Twenty-one cases were focally positive for EMA (21/30), 4 cases for SMA (4/23), 2 cases for S-100 (2/28), and 2 cases (2/35) for CKpan. Twenty-eight cases (28/28) had SS18 rearrangement displaying a split signal on FISH analysis. Eight cases were found to have mutations in SMC1A, NOTCH2, CDK12, SPRY4, BRCA1, STK11, NF2, and PDGFRα genes using NGS. Eighteen of the 29 patients survived and 16 showed disease progression. <b>Conclusions:</b> PTSS is more commonly found in the lungs than other sites and has non-classical morphological features of various types, which need to be differentiated from other tumors. TRPS1 is highly expressed in PTSS and has certain diagnostic values. The diagnosis of PTSS also requires combination of patient's medical history with thorough imaging studies.