Stereotactic Body Radiotherapy for the Treatment of Distant Metastases in Thyroid Cancer: A Phase II Prospective Clinical Study

Inclusion Criteria: 1. Participants with primary lesion pathologically confirmed as thyroid cancer, and distant metastatic lesions confirmed either by pathology or by two types of imaging modalities. Based on the investigator's judgment, the participant's distant metastatic lesions are expected to be tolerable to Stereotactic Body Radiotherapy (SBRT). The distant metastases should be oligometastatic (total number of metastatic lesions ≤ 5). 2. Participants who have failed prior systemic therapy. Systemic therapy includes radioactive iodine therapy, immunotherapy, targeted therapy, clinical trial therapy, etc. 3. Age: 18 - 70 years old. 4. KPS ≥ 70. 5. Expected survival period ≥ 3 months. 6. Before treatment, neutrophils ≥ 1.6 × 109/L, hemoglobin ≥ 80g/L, platelets ≥ 80×109/L; creatinine clearance rate ≥ 60 ml/min; non-hepatic metastasis patients (total bilirubin ≤ 1.5 times the upper limit of normal, ALT ≤ 1.5 times the upper limit of normal, AST ≤ 1.5 times the upper limit of normal), hepatic metastasis patients (total bilirubin ≤ 3 times the upper limit of normal, ALT ≤ 3 times the upper limit of normal, AST ≤ 3 times the upper limit of normal, normal coagulation function, Child-Pugh grade A or B, normal liver tissue volume \> 700 mL) 7. Signed informed consent form and willing to complete the study according to the protocol. Exclusion Criteria: 1. Patients who achieved complete remission after comprehensive treatment. 2. Pathologically confirmed as a second primary tumor. 3. Received radiotherapy to the corresponding area and the radiotherapy site overlapped with this one. 4. Uncontrollable malignant pleural or peritoneal effusion. 5. Severe internal medical complications and contraindications to radiotherapy. 6. Pregnant women of childbearing age with positive pregnancy test or lactating women. 7. Active mental disorders or other psychological disorders that affected the patient's signing of informed consent and understanding ability. 8. Uncontrollable infections or active infections, such as immunodeficiency or AIDS patients.