Primary Synovial Sarcoma of the Thyroid Gland Presenting With Early Pulmonary Metastasis: Diagnostic Pathway, SS18 Gene Fusion Confirmation, and Clinical Course

Objective:Rare disease Background:Primary synovial sarcoma of the thyroid gland (PSST) is an exceptionally rare and aggressive neoplasm.Twentyone cases have been reported in the literature to date, making this the 22nd reported occurrence.This report presents the case of a 47-year-old woman with PSST confirmed by SS18 gene fusion on fluorescence in situ hybridization (FISH), who subsequently developed pulmonary metastasis and doxorubicin-induced congestive heart failure, illustrating the aggressive clinical course of this entity. Case Report:We report the case of a 47-year-old woman presenting with dysphagia and a left-sided neck mass.Initial FNA was non-diagnostic; repeat FNA raised concern for a spindle cell neoplasm.Left hemithyroidectomy was performed with negative margins, and SS18 gene fusion was confirmed by FISH, establishing the diagnosis of PSST.Following multidisciplinary review, adjuvant chemotherapy was deferred and neck radiation was administered.Pulmonary metastasis was discovered 6 months postoperatively.The patient received eribulin mesylate and subsequent palliative doxorubicin, and her case was complicated by doxorubicin-induced congestive heart failure.She subsequently developed progressive systemic metastatic disease, including brain and bone involvement, and died of the disease 65 months after initial presentation. Conclusions:This case demonstrates that SS18 FISH confirmation is essential when FNA is non-diagnostic for a thyroid spindle cell mass.Despite negative margins and adjuvant radiation, early pulmonary metastasis and ultimately fatal systemic metastatic disease to the brain and bone occurred, underscoring the aggressive clinical course of PSST.Doxorubicin-induced cardiac toxicity further illustrates the systemic therapy burden.BCL-2-directed strategies remain investigational.Complete surgical resection with negative margins is the cornerstone of management.