Primary Synovial Sarcoma of the Thyroid: Radiological and Pathological Correlation of a Rare Malignancy

Synovial sarcoma is a rare, high-grade soft tissue sarcoma that typically arises near the large joints of the extremities, and primary involvement of the thyroid gland is exceptionally uncommon.We report a 27-yearold man who presented with an anterior neck mass that had enlarged over 11 months, with dysphagia, dyspnea, cough, and hoarseness for three months.Ultrasound showed a 13×10 cm heterogeneous, vascular, solid-cystic right thyroid lobe mass with coarse calcifications and heterogeneous stiffness on shear elastography.Contrast-enhanced computed tomography (CT) showed a large, heterogeneously enhancing mass that infiltrated the right internal carotid artery, encased the external carotid artery, and displaced the trachea, together with multiple pulmonary metastases and bilateral hilar lymphadenopathy but no cervical lymphadenopathy.Biopsy showed a biphasic tumor that was positive for pan-cytokeratin, epithelial membrane antigen, and transducin-like enhancer of split 1 and negative for thyroid transcription factor-1, thyroglobulin, calcitonin, and PAX8.Fluorescence in situ hybridization confirmed the SS18-SSX fusion, establishing the diagnosis of primary synovial sarcoma of the thyroid.Because the tumor infiltrated the carotid vessels and metastases were present, upfront surgery was considered unsafe, and systemic chemotherapy was started.The patient was then lost to follow-up.Primary synovial sarcoma of the thyroid should be considered in a young adult with a rapidly enlarging, vascular, calcified solid-cystic thyroid mass, particularly when pulmonary metastases occur without cervical nodal disease.Imaging can suggest the diagnosis and define resectability, but histology, immunohistochemistry, and molecular testing are needed for confirmation.