Objective: To explore the predictive value of MR-DWI for the assessment of radiosensitivity in nasopharyngeal carcinoma(NPC).Methods: A prospective cohort study was performed in a single hospital.Patients with NPC who received intensity-modulated radiotherapy (IMRT) with or without chemotherapy were enrolled from April 2010 to November 2011.A Signa 1.5T EXCITE Ⅲ HD magnetic resonance imaging device was used.MRI scans were performed within the week before the treatment,two weeks after the start of radiotherapy, and three months after the end of radiotherapy.The primary tumor apparent diffusion coefficient (ADC) was measured before treatment (ADC0) andtwo weeks after the start of IMRT (ADC1).Change in ADC(△ADC) was calculated as follows: [(ADC1-ADC0)/ADC0]× 100.The short-term effect of radiotherapy was assessed three months after the end of radiotherapy using the WHO response evaluation criteria in solid tumors.Patients diagnosed with no residual tumor had to meet both of the following response evaluation criteria: (1) MRIexamination suggesting no evident soft tissues in the nasopharynx or no local bulges due to thickening of the mucous membrane of the nasopharynx; and(2) nasopharyngoscopy indicating a smooth mucous membrane,no evidence of soft tissues or rough mucous membrane, and no cancer cells detected on biopsy.Patients diagnosed with a local residual tumor met either of these two criteria: (1) MRI examination suggesting residual soft tissues or the thickening of the mucous membrane of the nasopharynx with local bulges; or(2) pathological examinations detecting cancer cells on biopsy of the nasopharynx.