Camrelizumab After Completion of Radiotherapy for Oligometastatic Nasopharyngeal Carcinoma

Inclusion Criteria: * Patients with histologically confirmed non-keratinizing (according to World Health Organization (WHO) histologically type). * Patients with oligometastatic NPC (defined as ≤5 metastases, ≤2 organs), without local recurrence of nasopharynx. * Completion of radiotherapy 4-12 weeks prior to enrollment. At list one cycle of adjuvant platinum-based chemotherapy. * Satisfactory performance status: Karnofsky scale (KPS) \> 70. * Adequate marrow: leucocyte count ≥4000/μL, hemoglobin ≥90g/L and platelet count ≥100000/μL. * Normal liver function test: Alanine Aminotransferase (ALT)、Aspartate Aminotransferase (AST) \<1.5×upper limit of normal (ULN) concomitant with alkaline phosphatase (ALP) ≤2.5×ULN, and bilirubin ≤ULN. * Adequate renal function: creatinine clearance ≥50 ml/min. * Expected lifetime over 3 months * Patients must be informed of the investigational nature of this study and give written informed consent. Exclusion Criteria: * Previous history of autoimmune disease or unstable autoimmune disease. * Prior malignancy except adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer. * Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-Cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) antibody (including ipilimumab or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways). * Active autoimmune disease requiring systemic treatment within the past 3 months or a documented history of clinically severe autoimmune disease, or a syndrome that requires systemic steroids or immunosuppressive agents. Subjects with vitiligo or resolved childhood asthma/atopy are an exception to this rule. Subjects that require intermittent use of bronchodilators or local steroid injections are not excluded from the study. Subjects with hypothyroidism stable on hormone replacement or Sjorgen's syndrome are not excluded from the study. * Any severe intercurrent disease, which may bring unacceptable risk or affect the compliance of the trial, for example, unstable cardiac disease requiring treatment, renal disease, chronic hepatitis, diabetes with poor control (fasting plasma glucose \>1.5×ULN), and emotional disturbance.