Synovial sarcoma is a mesenchymal tumor that accounts for 5%‐10% of soft tissue sarcomas (Kirkham et al. 2020; Nowicki et al. 2017). It primarily affects the limbs, with more rare occurrences in the heart.

The average age of patients was 32.5 years, with a higher prevalence in males (3.5:1 male to female ratio) (Hannachi Sassi et al. 2004). Patients commonly present with atypical clinical manifestations such as palpitations and dyspnea, which can often lead to misdiagnosis. Tumors can be detected using computed tomography (CT), magnetic resonance imaging (MRI), and cardiac ultrasound. The majority of cases (71%) are found on the right side of the heart, particularly in the right atrium (Wang and Li 2013), while only a small percentage (8.6%) occur in the left atrium (Coli et al. 2019).Our example came from the right interatrial septum, which is even more unusual. Cancer thrombosis on the right side of the heart increases the risk of pulmonary embolism. This patient’s tumor was big, spanning from the right interatrial septum to the right atrium and right ventricle, with the right atrium taking up the majority. A preoperative chest CT revealed many tiny nodules in both lungs. During the operation, it was discovered that the tumor had a loose texture, increasing the danger of tumor embolus dissociation during resection. As a consequence, the patient experienced pulmonary embolism and bilateral lung metastases.

Histologically, synovial sarcoma is a monomorphic spindle-cell sarcoma with variable epithelial differentiation. It can be categorized into three types: monophasic, biphasic, or poorly differentiated. The monophasic variety is made up entirely of spindle cells, whereas biphasic synovial sarcoma has both epithelial and spindle cell components (Jo and Fletcher 2014).Cardiac synovial sarcoma exhibits similar histological morphology. (Coli et al. 2019) retrospectively found that monophasic forms were approximately twice as common as biphasic forms in cardiac synovial sarcoma. Under the microscope, the patient was primarily made up of spindle cells, and the appearance was nearly identical. There were no zones of calcification or ossification, indicating a monophasic type. The cell nuclei were relatively uniform in shape, being spindle-shaped or oval and mostly overlapping, with darkly pigmented nuclei, indistinct nucleoli, and little or unclear cytoplasm.

There are no particular immunological markers for synovial sarcoma. The more sensitive markers are EMA, CK, CD99 (Bishop et al. 2015), Bcl-2, TLE-1 (Marchione et al. 2020), Calponin (Fisher et al. 2003), and Vimentin (Teng et al. 2021). A study reveals that H3K27Me3 and SOX10 are effective markers for neural soft tissue cancers (Sbaraglia and Dei Tos 2019). In this case, the absence of H3K27Me3, SOX10, and S100 can help exclude malignant peripheral nerve sheath tumors. Additionally, the lack of MyoD1 and S100 expression eliminates the possibility of rhabdomyosarcoma (Lak et al. 2021) and melanoma (Neubert et al. 2018), respectively, as these markers are known to be sensitive to those conditions. The patient has positive expression of TLE-1, Calponin, and EMA, supporting the diagnosis of synovial sarcoma.

According to molecular genetics, 95% of synovial sarcomas have a characteristic translocation of chromosome t (X: 18). The fusion of the SS18 (SYT) gene with one of the SSX genes (SSX1, SSX2, or SSX4) causes disease development. When synovial sarcoma is considered by immunohistochemistry and SS18 translocation is detected by FISH, synovial sarcoma can be diagnosed (Su et al. 2021).

The long-term survival percentage for primary cardiac malignant tumors is extremely low. According to the research of (Sultan et al. 2020), the 1-year and 5-year survival rates for primary cardiac malignant tumors were 45.3% and 11.5%, respectively, based on the US NCDB oncology database. Compared to the non-surgical group, the surgery group had considerably higher long-term survival rates (p < 0.0001).Cardiac synovial sarcoma is a highly malignant tumor with a lower incidence rate. There are currently no standardized treatment guidelines. Retrospective study (Coli et al. 2019) reported that the median survival time of 55 patients with cardiac synovial sarcoma was 27 months. Age, complete resection, and adjuvant chemoradiation were independent prognostic factors.The patient was found to have lung metastases. Due to the large size of the tumor and the risk of cardiac tamponade, palliative resection of the primary tumor is the only option to reduce the occurrence of severe complications.The most prevalent chemotherapy regimens are ifosfamide and doxorubicin; however, their efficacy is limited. In this example, the patient’s lung lesions grew slowly following chemotherapy. NGS testing reveals KDR amplification, which may result in overexpression of the VEGFR2 protein. We changed to anlotinib, which may have been sensitive to antivascular inhibitor medication, but the outcomes were unfavorable. As the disease advanced, pulmonary metastases infiltrated the major vessels near the right hilum of the lung, leading to massive hemoptysis and ultimately death. The patient had a survival time of 8 months with poor tolerance to chemotherapy, which was significantly lower than the median survival time reported by Coli et al. (27 months). It is important to note that the cases documented by Coli et al. predominantly did not involve distant metastasis and were successfully treated with complete resection, leading to a longer median survival time.

Here, We document a case of primary pericardial synovial sarcoma originating from the atrial septum and associated pulmonary embolism in a 60-year-old woman. This patient had pulmonary metastasis and pulmonary embolism at the time of diagnosis. He was in stage IV and theoretically lost the chance of surgery. However, due to the location of the primary tumor in the heart and the potential risks associated with blood flow obstruction, pulmonary embolism, and hemorrhage, a palliative tumor resection was performed. Given the intricate nature of cardiac synovial sarcoma and the lack of effective treatment options for patients in advanced stages, anti-angiogenic therapy continues to offer valuable insights into combating the disease. This emphasizes the necessity of exploring diverse treatment modalities to improve patient outcomes and potentially discover new avenues for therapeutic intervention. Research in this area remains crucial for advancing our understanding of the disease and developing more effective treatment strategies.