Similar experience —> I only had 1 day of adverse side-effects during my 28 sessions of proton beam radiation treatments (during April-May 2021).
On the 3rd day of treatment, I had urinary issues. My RO told me that with some men there’s an inflammatory response to the radiation, and if that inflammation is near the urethra can cause the issues that I was experiencing.
He recommended that I take 2 Tamsulosin/day for the remainder of the treatments.
Everything cleared up by the next day; I haven’t had any issues since.