Why might urinary symptoms occur during radiation?

The bladder receives incidental radiation because it is located next to the prostate

The prostate is anatomically situated directly beneath the bladder. During radiation therapy, even with advanced precision, the bladder inevitably receives some incidental radiation exposure. This radiation can cause inflammation and irritation in the bladder tissue, which frequently leads to common side effects such as increased urinary frequency, a sense of urgency, or a burning sensation during urination as the body responds to the treatment.