Camrelizumab Plus Stereotactic Body Radiotherapy vs Camrelizumab Alone For Oligometastatic Nasopharyngeal Carcinoma

Inclusion Criteria: 1. Male or female; 18-70 years of age. 2. Primary lesion and regional lymph nodes completed radical radiotherapy 3 months before stereotactic body radiotherapy and diseases well controlled. 3. Underwent at least first-line systemic chemotherapy, regardless of regimen and curative effect. 4. Imageological evidence for oligometastatic lesions (metastatic tissue biopsy preferred but not necessary). 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. ECOG performance status of 0 or 1. 6. Stereotactic body radiotherapy applicable for all metastatic lesions according to MDT. 7. If metastatic lesions have received local treatment (surgery, radiofrequency ablation, radiotherapy etc.): * Eligible if treated lesion is well controlled according to imageological examinations, and the lesion does not need stereotactic body radiotherapy. * If treated lesion is not controlled according to imageological examinations: * Eligible if the treatment is surgery and that stereotactic body radiotherapy is applicable for the treated lesion. * Ineligible if the treatment is radiofrequency ablation or radiotherapy. 8. Maximum diameter of brain metastatic lesion no more than 3cm. 9. 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. 10. Life expectancy more than 12 weeks. Exclusion Criteria: 1. Immunotherapy (PD-1/PD-L1 or CTLA-4 monoclonal antibody) failure. 2. CHD no less than grade 2, arrhythmia (QTc interval over 450ms for male and 470ms for female) or cardiac insufficiency. 3. History of severe hypersensitivity to any ingredient of PD-1/PD-L1 or other monoclonal antibody. 4. chemotherapy (cytotoxic or molecular targeted) within 4 weeks before stereotactic body radiotherapy. 5. Imageological evidence for spinal cord compression, or tumor less than 3mm away from spinal cord. 6. Patient with brain metastasis who needs decompression surgery. 7. Other malignancy or malignant hydrothorax. 8. Concurrent known or suspicious autoimmune disease, including dementia and epilepsy. 9. Use of large dose corticosteroids within 4 weeks before study drug administration. 10. Concurrent medical condition requiring the use of immunosuppressive medications, or immunosuppressive doses of systemic or absorbable topical corticosteroids. 11. Active tuberculosis (TB), anti-TB treatment is ongoing or within 1 year prior to screening 12. 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. 13. 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. 14. Received any anti-infective vaccine (e.g. influenza vaccine, varicella vaccine, etc.) within 4 weeks prior to enrollment. 15. Pregnancy or lactation. 16. Other ineligible patients according to attending doctor.