Abstract:

Thyroid tumors are mainly classified into epithelial and non-epithelial types， among which epithelial tumors account for 95%-99% and originate from follicular epithelial cells. Non-epithelial tumors mostly arise from lymphoid tissue， and a very small proportion originate from mesenchymal tissue， also known as mesenchymal tumors， accounting for less than 0.5%. Clear cell sarcoma of soft tissue （CCSST）， as a special type of mesenchymal tumor， is exceptionally uncommon and associated with an unfavorable prognosis. This article reported a case of primary thyroid CCSST in a patient whose tumor was confirmed by genetic testing to carry EWSR1 gene rearrangement （22q12 break）， and the relevant literatures were reviewed to summarize the clinical characteristics and experience in diagnosis and treatment. The patient， a 57-year-old female， underwent regular follow-up after radionuclide therapy for hyperthyroidism more than 10 years ago. A thyroid nodule was detected during the physical examination one year ago， and after further evaluation， she was admitted for total bilateral thyroidectomy. Postoperative paraffin pathology and immunohistochemistry showed that the tumor cells were positive for S-100 and sex-determining region Y-box 10（SOX10） and negative for human melanoma black 45（HMB45）. Fluorescence in situ hybridization （FISH） confirmed the presence of EWSR1 gene rearrangement （22q12 break）， leading to a definite diagnosis of primary thyroid CCSST with EWSR1 gene rearrangement. Postoperative positron emission tomography-computed tomography （PET-CT） excluded metastasis elsewhere in the body. In clinical practice， rare genetic alterations， such as EWSR1 rearrangement， should be carefully considered in the patients with unusual thyroid tumors. Individualized diagnosis and therapeutic strategies， together with long-term follow-up schedules should be formulated to achieve early identification and precise intervention， thereby improving the long-term prognosis of patients.

Key words: Thyroid mesenchymal malignancy, Clear cell sarcoma of soft tissue, Primary tumor, Thyroid, EWSR1 gene, Case report