Hello, Although disturbed bowel movements ( diarrhoea ) for few weeks are common after external radiation therapy for prostate cancer,but in few cases radiation may cause permanent problems like proctitis ( inflammation of rectum ) with pain in anus and bowel disturbance like diarrhoea. Treatment of diarrhoea involves use of anti diarrheals and bulking agents like FYBOGEL. It is advised to avoid high fibre diet. For pain,anti inflammatories are prescribed,however your doctor would be in best position to advise and guide you according to your symptoms. Thanks