@knoyes01 Radiation these days is a LOT more targeted than in the past. Salvage radiation uses computer assisted guidance to avoid (as much as possible) areas of healthy tissue such as bladder and rectum.
A simulation is done under ideal conditions - full bladder/empty rectum. The Xray computer program will scan you before each session to be sure that your internal organs are in the SAME position as the simulation in order to keep the beams focused only where the RO wants them.
Your only job is to follow the bladder/rectum protocol laid out by your team. You will be fine.
Phil