Background: Synovial sarcoma, generally known as a soft tissue tumor, can also occur in the head and neck region, including the thyroid gland. Cytologic findings are important to differentiate the tumor from other types of neoplasms arising in the thyroid gland.
Case: A 60-year-old man complained of hoarseness. A palpable neck tumor was detected, and a computed tomography scan showed a thyroid tumor accompanied by destruction of the thyroid and cricoid cartilage. The results of a preoperative fine needle aspiration biopsy showed numerous spindle cells with pale cytoplasm and oval nuclei with fine, granular chromatin, all of which suggested a medullary carcinoma. The extirpated thyroid tissue weighed approximately 120 g, and a grayish white, elastic, solid tumor (6.8 x 6.5 cm) was present in the left lobe. Histologically, fasciculation of spindle cells that had proliferated solidly and densely was observed. Also, the expression of a chimera gene, SYT-SSX, was detected in the tumor tissue.
Conclusion: Synovial sarcoma of the thyroid is extremely rare, and its diagnosis by fine needle aspiration biopsy is generally considered very difficult. The detailed cytologic findings observed here might be helpful with the differential diagnosis of thyroid neoplasms.