Objective:To analyze the effects of radiotherapy on the onset and progression of mastoiditis in patients with nasopharyngeal carcinoma (NPC) using magnetic resonance imaging (MRI) and to explore the risk factors for the onset of mastoiditis after radiotherapy.Methods:The onset and progression of mastoiditis of 204 NPC patients 3, 12, and 24 months after radiotherapy were analyzed based on MRI images. The multi-factor logistic regression analysis was applied to explore the risk factors of the onset of mastoiditis after radiotherapy. The cross-sectional area of the tensor veli palatini muscle was measured and the relationship between the atrophy degrees of the tensor veli palatini muscle and the onset of mastoiditis was analyzed.Results:The incidence of mastoiditis before radiotherapy was 20.6% (84/408, ears), and was 41.1% (168/408, ears), 22.3% (91/408, ears), and 19.6% (80/408, ears), respectively 3, 12, and 24 months after radiotherapy. The incidence of radiotherapy-induced mastoiditis was 35.8% (116/324, ears), 18.2% (59/324, ears), and 16.4% (53/324, ears), respectively 3, 12, and 24 months after radiotherapy. The remission rate of 63 patients (83 ears) who developed mastoiditis 3 months after radiotherapy was 63.9% (53/83, ears) and 75.9% (63/83, ears), respectively 12 and 24 months after radiotherapy. The remission rate of 54 patients (60 ears) who suffered mastoiditis before radiotherapy was 43.3% (26/60, ears), 65.0% (39/60, ears), and 73.3% (44/60, ears) 3, 12, and 24 months after radiotherapy. The multivariate analysis showed that the independent risk factors for radiotherapy-induced mastoiditis included age ≥50, clinical stages Ⅲ-Ⅳ, radiotherapy dose > 70 Gy, and tumors invading pharyngeal ostium of the eustachian tube. In addition, the atrophy degree of tensor veli palatini muscle 12 and 24 months after radiotherapy correlated with the onset of mastoiditis.Conclusions:The incidence of mastoiditis significantly increased 3 months after radiotherapy and significantly decreased 12 months after radiotherapy for NPC patients. The natural remission rate of radiotherapy-induced mastoiditis 12 months after radiotherapy was over 70%. The independent risk factors for radiotherapy-induced mastoiditis include age ≥50, clinical stages Ⅲ-Ⅳ, radiotherapy dose >70 Gy, and tumor invading pharyngeal ostium of the eustachian tube. The atrophy degree of the tensor veli palatini muscle 12 and 24 months after radiotherapy correlates with the onset of mastoiditis.