摘要：Abstract Background and purpose Local recurrences after radiotherapy are dose-dependent and occur in the dominant intraprostatic lesion (DIL). The purpose of this study was to evaluate the impact of a simultaneous integrated boost (SIB) to the magnetic resonance imaging (MRI)-defined DIL on toxicity. Materials and methods Four-hundred and ten patients were treated with intensity-modulated radiotherapy. A median dose of 78 Gy was prescribed to the prostate. A SIB of 82 Gy to the DIL was performed in 225 patients (SIB+). Genitourinary and rectal toxicity on fixed time points up to 8 years were compared between SIB− (185 patients) and SIB+ patients. Chi-square, Fisher’s exact and Kaplan–Meier statistics were applied. With a median follow up of 72 months, the six-year actuarial risk of genitourinary and rectal toxicity grade ⩾ 2 was 31% and 12% respectively. The actuarial risk of developing toxicity and incidence of symptoms at fixed time points were not increased with a SIB. Conclusion Performing a SIB did not increase genitourinary or rectal toxicity up to 8 years’ follow-up.