Combining RT With Immunotherapy and Chemotherapy in Metastatic Nasopharyngeal Carcinoma

Inclusion Criteria: * Age:18-75 years, male or female. * ECOG 0-2 * Histologically or cytologically confirmed de novo metastatic nasopharyngeal carcinoma.(stage IVb, AJCC 8th) * Complete response or partial response after at least 3 cycles (no more than 6 cycles) of chemotherapy combined with immunotherapy * Measurable disease based on Response Evaluation Criteria In Solid Tumors (RECIST) 1.1. * Adequate organ function. * Patient has given written informed consent. Exclusion Criteria: * Unwilling or unable to provide informed consent * Intolerance to radiotherapy or immunotherapy * Patients who have head and neck radiotherapy history. * previous or recent another malignancy, except for nonmelanoma skin cancer or cervical cancer in situ * women in pregnancy, lactation period, or no pregnancy test 14 days before the first dose * in other clinical trials within 30 days * Patients with autoimmune disorder, including but not limited to systemic lupus erythematosus or multiple sclerosis; * History of primary immunodeficiency * History of active tuberculosis, drug-induced interstitial lung disease, or ≥ Grade 2 pulmonitis; * Patients with human immunodeficiency virus (HIV) positive; * Comorbidities that cannot be controlled by concomitant treatment, including but not limited to: ongoing or active infection, unexplained fever \> 38.5°C (subjects with neoplastic fever are judged by the investigator to be included), symptomatic congestive heart failure ≥ Grade 2 according to New York Heart Association (NYHA) functional classification, LVEF (left ventricular ejection fraction) \< 50%, hypertension poorly controlled by drugs, unstable angina, arrhythmia, active peptic ulcer disease or gastritis; * not suitable for this study judged by researchers