The investigators aim to explore a new EBV DNA surveillance method with both high sensitivity and specificity in nasopharyngeal carcinoma (NPC) patients. the investigators aim to conduct plasma EBV DNA counting by next generation sequencing (NGS) in non-metastatic NPC patients on their diagnose, after two cycles of induction chemotherapy (IC), and 4-8 weeks after definitive radiotherapy. The investigators aim to explore whether sequencing-based counting is better than PCR analysis in plasma EBV-DNA surveillance, so as to monitoring tumor responses to treatment and for guiding individualized treatment adaptation in the future.