Whole-target Consolidation Therapy Under Systemic Therapy for Oligometastatic Nasopharyngeal Carcinoma

Inclusion Criteria: 1. Male or female; 18-70 years of age. 2. Had histopathologically confirmed nonkeratinizing metastatic NPC that was diagnosed as stage IVb NPC (AJCC, 8th; the metastatic tissue biopsy is preferred, not necessary). 3. Patients who had not received anti-tumor therapy for nasopharyngeal cancer before this clinical trial. 4. Patients evaluated to have a partial response (PR) or stable disease (SD) by head and neck MRI and PET/CT after 3 months of locoregional radiotherapy, and the metastatic lesions were assessed as oligometastatic lesions (the number of total metastatic lesions no more than 5 and the number of metastatic lesions within a single organ no more than 3). 5. Stereotactic body radiotherapy applicable for all metastatic lesions according to MDT. 6. ECOG performance status of 0 or 1. 7. Maximum diameter of brain metastatic lesion no more than 3cm. 8. Maximum diameter of metastatic lesion (brain excluded) no more than 5cm. * Maximum diameter of bone metastatic lesion no more than 6cm if attending doctor decides it is safe to apply the treatment. 9. Life expectancy more than 6 months. Exclusion Criteria: 1. History of severe hypersensitivity to any ingredient of PD-1/PD-L1 or other monoclonal antibody. 2. chemotherapy (cytotoxic or molecular targeted) within 4 weeks before stereotactic body radiotherapy. 3. Imageological evidence for spinal cord compression, or tumor less than 3mm away from spinal cord. 4. Patient with brain metastasis who needs decompression surgery. 5. Other malignancy or malignant hydrothorax. 6. Concurrent known or suspicious autoimmune disease, including dementia and epilepsy. 7. CHD no less than grade 2, arrhythmia (QTc interval over 450ms for male and 470ms for female) or cardiac insufficiency. 8. Use of large dose corticosteroids within 4 weeks before study drug administration. 9. Concurrent medical condition requiring the use of immunosuppressive medications, or immunosuppressive doses of systemic or absorbable topical corticosteroids. 10. Active tuberculosis (TB), anti-TB treatment is ongoing or within 1 year prior to screening 11. Subjects with any active autoimmune disease or history of autoimmune disease, or history of syndrome that requires systemic steroids or immunosuppressive medications, including but not limited to the following: rheumatoid arthritis, pneumonitis, colitis (inflammatory bowel disease), hepatitis, hypophysitis, nephritis, hyperthyroidism, and hypothyroidism, except for subjects with vitiligo or resolved childhood asthma/atopy. 12. Has a known history of human immunodeficiency virus (HIV), has hepatitis B surface antigen (HBsAg) positive with hepatitis B virus (HBV) DNA copy number of ≥1000cps/ml or hepatitis C virus (HCV) antibody positive. 13. Received any anti-infective vaccine (e.g. influenza vaccine, varicella vaccine, etc.) within 4 weeks prior to enrollment. 14. Pregnancy or lactation. 15. Other ineligible patients according to attending doctor.