In order to improve locoregional control and to eradicate micrometastases for patients with locally advanced nasopharyngeal carcinoma (NPC), induction chemotherapy followed by concurrent chemoradiotherapy has been the preferred therapeutic approach.Although it would be of great advantage to identify non-responder during or immediately after therapy to formulate an alternative treatment strategy, non-invasive methods for early evaluation of therapeutic response for advanced NPC patients have not been fully evaluated. Decrease of tumor volume in anatomical images and relief of symptoms in clinical follow-up have been used in many conventional approaches to evaluate the effectiveness of therapy. However, an effective therapy could very well have resulted in early subclinical alterations in tumor physiology and biochemistry. These alterations may have occurred long before changes in the tumor mass become apparent. Thus therapeutic effect may not be assessed accurately using the conventional anatomical imaging studies, such as CT and MRI.Recent studies have shown that 18F-FDG PET, a whole-body imaging technology based on glucose metabolism, can effectively detect subclinical and clinical therapeutic responses at stages that are earlier than those detected by conventional approaches.We therefore propose to conduct a prospective study to evaluate therapeutic response using 18F-FDG PET before and after induction chemotherapy for locally advanced NPC patients.