"Wash your skin gently with soap and water and gently pat dry. Avoid shaving and don't use heat and cooling pads/ice, or wax and creams for hair removal in the treatment area. Men can use an electric trimmer instead of shaving."
While I hope that my body is reasonably clean at any time, I certainly wasn't given any instructions about washing my pelvic area before radiotherapy sessions. Further, the radiographers never examined the relevant area before treating me to check for cleanliness. (It was covered by a sort of paper towel throughout).
Given the natural variations that occur between sessions along the path of the beam (such as differences in bladder size and contents, as well as changes in body fat over time), it's hard to see how a bit of grime might make much difference really.