It's strange how RT can lead to Diarrhea or less usual but happened in my case, constipation. I think diet can make quite a difference. In either case you can have a sore bum. The advice I was given was to gently dab (not rub) Bepanthen cream on the affected area. This is a very mild cream developed for babies and sensitive skin. Radiologists also recommend other creams. Strong and scented soaps should be avoided. The situation should have started to improve by 6 weeks but there is always a risks of an instance of Diarrhea or Prostatitis at times over months after RT was given.