Primary Synovial Sarcoma of the Thyroid: Challenges in Cytologic Diagnosis and Review of the Literature.

BACKGROUND

Primary synovial sarcoma (SS) of the thyroid (PSST) is extremely rare. Its differential diagnosis from other neoplasms is essential since it has different management protocols and prognosis.

CASE: A 26-year-old man with a 4.5-cm solid lobulated mass was seen at an outside hospital. Fine needle aspiration (FNA) was interpreted as a papillary carcinoma, and a total thyroidectomy was performed. The final histologic diagnosis was spindle epithelial tumor with thymus-like differentiation (SETTLE). No metastases were detected at that time, and the patient received radioactive iodine treatment. Two years post-surgery, he was seen at our hospital with a local recurrence, and FNA was considered as consistent with SETTLE. The mass was resected, and a left modified radical neck dissection was performed. The tumor revealed necrosis and a high mitotic index. Following histologic, immunohistochemical, and molecular studies, the tumor was classified as a PSST. The patient received chemotherapy and targeted immunotherapy, but he died 41 months after the initial presentation.

CONCLUSION

The main diagnostic pitfall of PSST is SETTLE. The presence of mitotic figures and basal lamina material, negative staining for smooth muscle actin, and positive staining for transducer-like enhancer of split 1 antibody favor SS over SETTLE. SYT gene rearrangement is essential to establish the definitive diagnosis of PSST.