| Summary: | Purpose: This study aimed to clarify the predictive factors for otitis media with effusion (OME) due to Eustachian tube dysfunction in patients treated with carbon-ion radiation therapy (CIRT) for head and neck cancers. Methods and Materials: We investigated patients with head and neck cancer whose Eustachian tube was irradiated by CIRT between October 2013 and December 2018 at our institution. OME severity was assessed by the proportion of mastoid cell opacification of magnetic resonance or computed tomography imaging (grade 0: <5% of volume of mastoid cell with opacification by fluid collection; grade 1: 6%-33%; grade 2: 34%-67%; and grade 3: 68%-100%). Clinical factors and dosimetric parameters affecting the development of grade 2 to 3 OME were analyzed using a log-rank test and Cox proportional hazards model. Results: In total, 141 patients were analyzed. The median follow-up period was 25.2 months. Grade 2 to 3 OME was observed in 65 patients, with a median incidence period of 6.5 months. According to the multivariate analysis, the mean dose of the cartilage part was a significant independent predictive parameter of grade 2 to 3 OME. The 2-year incidence rate of patients with a mean dose of the cartilage part of <40.59 Gy (relative biological effectiveness) and ≥40.59 Gy (relative biological effectiveness) was 24.2% (95% confidence interval, 15.1%-37.4%) and 66.4% (95% confidence interval, 54.5%-78.0%), respectively. Conclusions: Our findings may be useful to predict the risk of grade 2 to 3 OME due to Eustachian tube dysfunction before CIRT.
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