Sintilimab Combined With Chemotherapy and SBRT in Limited Metastatic Head and Neck Squamous Cell Carcinoma (LM-HNSCC)

Inclusion Criteria: * Sign written informed consent before implementing any trial-related procedures; * Male or female, age ≥18 years old; * Histologically confirmed head and neck squamous cell carcinoma, which was diagnosed as initial stage IVC/M1(synchronous metastatic disease) according to the 8th edition of UICC/AJCC or previously treated head and neck tumor with newly occurred metachronous metastatic disease ; * Pathological diagnosis of metastasis is not mandatory, but the clinical diagnosis needs to receive consent of MDT; * The number of metastases is 1-10; * PD-L1 expression is positive, CPS≥1; * According to the evaluation criteria for the efficacy of solid tumors (RECIST version 1.1), at least one metastatic lesion is radiologically measurable; * Newly-diagnosed HNSCC who has not received any treatment previously or HNSCC who has been diagnosed metastases for the first time after treatment; * For patients who have received platinum-containing chemotherapy in the past, the interval between the new metastasis and the end of the last chemotherapy administration is at least 6 months; * After a comprehensive radiological examination, at least one extracranial metastatic lesion with a maximum diameter of ≤ 5cm (which can be treated with SBRT); * ECOG score 0-1 points; * Sufficient organ function, subjects need to meet the following laboratory indicators: In the past 14 days without using granulocyte colony stimulating factor, the absolute value of neutrophils (ANC) ≥ 1.5x109/L. In the case of no blood transfusion in the past 14 days, platelets ≥100×109/L. In the past 14 days without blood transfusion or using erythropoietin, hemoglobin\>9g/dL；Total bilirubin≤1.5×upper limit of normal (ULN); or total bilirubin\>ULN but direct bilirubin≤ULN；Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are within ≤2.5×ULN;Serum creatinine ≤1.5×ULN and creatinine clearance rate (calculated by Cockcroft-Gault formula) ≥60 ml/min; Good coagulation function, defined as International Normalized Ratio (INR) or Prothrombin Time (PT) ≤ 1.5 times ULN;Normal thyroid function is defined as thyroid stimulating hormone (TSH) within the normal range. If the baseline TSH is outside the normal range, subjects whose total T3 (or FT3) and FT4 are within the normal range can also be included in the group;The myocardial enzyme spectrum is within the normal range (if the investigator comprehensively judges that the simple laboratory abnormality is not of clinical significance, it is also allowed to be included); * Expected survival\> 1 year; * At least one lesion is RECIST 1.1 assessable lesion; * No previous PD1 or PD-L1 inhibitor treatment history; * Without comorbidities that needs treatment of immunodepressant; * Male subjects and women of childbearing age must have contraception within 24 weeks from the start of the study to the last time of using the drug. Exclusion Criteria: * The primary site is squamous cell carcinoma of the nasopharynx or skin cancer. * The number of metastases\>10; * Patients who have been diagnosed with other malignant tumors within 5 years before the first administration and have not been cured (excluding radically cured skin basal cell carcinoma, skin squamous epithelial carcinoma, and/or radically resected carcinoma in situ); * Currently participating in interventional clinical research treatment, or received other research drugs or used research devices within 4 weeks before the first administration; * Have received the following therapies in the past: anti-PD-1, anti-PD-L1 or anti-PD-L2 drugs or for another stimulating or synergistic inhibition of T cell receptors (for example, CTLA-4, OX-40, CD137) drug; * Received systemic treatment with anti-tumor indications Chinese patent medicines or immunomodulatory drugs (including thymosin, interferon, interleukin, except for local use to control pleural fluid) within 2 weeks before the first administration; * An active autoimmune disease that requires systemic treatment (such as the use of disease-relieving drugs, glucocorticoids, or immunosuppressive agents) occurred within 2 years before the first administration. Alternative therapies (such as thyroxine, insulin, or physiological glucocorticoids for adrenal or pituitary insufficiency, etc.) are not considered systemic treatments; * Administration of systemic steroid (not include nasal spray, inhalation or other local approach) within 7 days before the first administration of Sintilimab or any other ways of immunosuppression; * Receiving Xenografts or allogeneic hematopoietic stem cell transplantation in the past; * Allergic to component of research drugs or adjacent; * HIV infection; * Untreated active hepatitis B (HBsAg positive and HBV-DNA copy number larger than upper limit of threshold); * Untreated active hepatitis C (HCV antibody positive and HCV-RNA larger than upper limit of threshold); * Inoculation with live vaccine within 30 days before the first administration of Sintilimab; * Women patients in the pregnancy or lactation period; * With severe or uncontrollable comorbidity.