Abstract: Objective To evaluate the role of diffusion-weighted imaging (DWI) and the value of apparent diffusion coefficient (ADC) in differential diagnosing necrosis and  recurrence of nasopharyngeal carcinoma after radiotherapy.Methods From Jul 2008 to Dec 2010，thirty-five patients were diagnosed postradiation nasopharyngeal necrosis，which were matched with the same number of patients randomly selected from the same period. All patients were confirmed pathologically，and received routine MRI and DWI (with the b values of 0s/mm² and 800s/mm²). ADC values of necrotic foci and parts of recurrence tumor were measured，and their mean ADC values were calculated.Results  Nasopharyngeal necrosis usually showed heterogeneity and low signal on DWI，but the recurrence group showed high signal.The mean ADC values in nasopharyngeal necrosis group and recurrence group were (1.073±0.0383)×10.3mm²/s and (0.844±0.0309)×10.3mm²/s respectively，the difference of which was statistically significant (t=4.645，P＜0.001). The maximal values in these two groups were (1.728±0.0527)×10.3mm²/s and (1.477±0.0675)×10.3mm²/s respectively，the difference of which was also statistically significant (t=2.932，P＜0.005). Conclusions DWI and ADC value are important in differential diagnosis of nasopharyngeal necrosis and tumor recurrence after radiotherapy.DWI and ADC can be used as important supplementary to routine MRI.

Key words: Nasopharyngeal carcinoma, Necrosis, Recurrence, DWI, ADC