Have completed 19 of 29 proton treatments and had Space OAR. Only preparation each day is filling bladder with water one hour before treatment. Most of the time I am in the range and maybe 4 times have had to drink more and wait 10- 20 minutes before treatment and a few times have had to drain some off. Slightly uncomfortable not being able to pee when you choose to but easily tolerated. I try to have a bowel movement before I go so as not to have excess gas ( eat prunes in the morning)that has to be released by the nurse. So far, I am batting 1,000.
Preparation is actually easily done.