Feb - June 2023, I had salvage radiation w/ short term ADT following prostatectomy in Aug 2022.
37 daily treatments IMRT over almost 8 weeks.
Radiation prep of almost full bladder and almost empty rectum were interesting experiences for me, and every man that I encountered at treatment.

It will be an individual plan for everyone; and a trial and error process.

Empty rectum/gas: My regular dietary goals are only whole grains and lots of vegetables.
I modified somewhat by avoiding/limiting the known gas producing foods such as brocolli and beans, and eating smaller salads.
Together with GasX at night, I managed to make it through treatment. Fortuitously, I am more regular than Amtrak.

Almost full bladder: Another individual regimine. I tried to drink 40 - 60 oz of water; urinate about 45 mins before treatment and then consume another 20 ozs or so of water, finishing 20 - 30 mins before treatment so that the bladder could refill.
It is an interesting tight rope to walk, and, again, every man had his own recipe.
I was kicked off the table one time for inadequate hydration.

And a major challenge is the occasional need to "release a little pressure" right before treatment.

A friend and I joked that the "perfect" radiation diet was a double whopper with cheese, no pickle or lettuce, and fries. 3 times a day.

And while you are in treatment remember "this too shall pass".

Best wishes to all.

And the gold ring is my PSA reported as undetectable at < .02 six and nine months post treatment, with a 3rd test this month🤞