A circulating tumor cell (CTC) count is an established prognostic factor in some malignancies such as metastatic breast cancer. However, the value of CTC in diagnosis and outcome prediction of metastatic nasopharyngeal carcinoma (mNPC) patients is not unknown. Through the observational prospective clinical trial, sensitivity, specificity, positive and negative predictive values of CTC in diagnosis of mNPC patients will be gained. Further, the value of CTC in outcome prediction of mNPC patients will be uncovered.