Don't worry, the radiotherapy staff are totally used to us patients struggling to manage the required states of bladder and rectum. And you will quickly work out which of the other men are also being treated for prostate cancer - they will be the ones going in then reappearing to drink more water, or pee out a required volume into a marked container, or do the job with the micro-enema.
In my experience the staff were very patient and understanding, and you got a bit of a community spirit from the fellow sufferers going through the same experience.
Good luck!