Primary synovial sarcoma of the thyroid mimicking medullary thyroid cancer

The differential diagnosis of a rapidly enlarging thyroid mass includes poorly differentiated thyroid carcinoma, anaplastic carcinoma, medullary thyroid carcinoma (MTC), lymphoma, and metastatic disease. Although fine-needle aspiration (FNA) provides important diagnostic clues, it may not be definitive. We report a 43-year-old woman with a long-standing goiter that enlarged rapidly over 6 months. FNA suggested MTC; however, serum calcitonin and carcinoembryonic antigen levels were within normal limits, creating clinicobiochemical discordance. Given this discrepancy, further evaluation was undertaken. Core needle biopsy of the thyroid mass demonstrated a spindle cell neoplasm, and immunohistochemistry supported a diagnosis of primary synovial sarcoma. Gallium-68-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-octreotate positron emission tomography-computed tomography revealed distant metastases involving the lungs, bones, and bone marrow. In view of advanced disease with vascular encasement and metastatic spread, neoadjuvant chemotherapy was initiated. This case highlights the importance of histopathologic confirmation in atypical presentations of thyroid malignancy and underscores the limitations of cytology in rare spindle cell tumors.