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Background/purpose: Post radiation mucosal ulcers (PRMU) after treatment for oropharyngeal squamous cell carcinoma (OPSCC) can have a huge negative impact on patients' quality of life, but little is known concerning risk factors and the impact of fraction size. Therefore, the goal of this study was to determine the pattern of PRMU development and to identify risk factors after a hypofractionated stereotactic body radiotherapy boost (SBRT) compared to conventionally fractionated radiotherapy for OPSCC., Material and Methods: We performed a retrospective cohort study ( N  = 332) of OPSCC patients with ≥ 1-year disease-free survival, treated with 46 Gy Intensity Modulated Radiotherapy (IMRT) (2 Gy fractions) followed by either an SBRT boost of 16.5 Gy (5.5 Gy fractions) ( N  = 180), or 24 Gy IMRT (2 Gy fractions) ( N  = 152). PRMU (grade ≥ 2) was scored when observed > three months after the last radiotherapy (RT) fraction (CTCAE v5.0). Potential risk factors were analyzed with Cox regression models using death as competing risk. Dose at the PRMU site was calculated by projecting delineated PRMU on the planning CT., Results: All cases of PRMU ( N  = 64) occurred within 24 months; all were grade 2. The cumulative incidence at 2 years in the SBRT boost group was 26% ( N  = 46) vs. 12% ( N  = 18) for conventional fractionation ( p  = 0.003). Most PRMU developed within nine months ( N  = 48). PRMU occurring > nine months ( N  = 16) were mainly observed in the SBRT boost group ( N  = 15). Sex ( p  = 0.048), acute tube feeding ( p = < 0.001), tumor subsite tonsil ( p  = 0.001), and N stage ( p  = 0.017) were associated with PRMU risk at multivariable regression in the hypofractionated SBRT boost group. All 25 delineated PRMU were located within the high dose regions., Conclusion: The risk of PRMU should be included in the cost benefit analysis when considering future research using a hypofractionated SBRT boost for OPSCC patients.