In my opinion, it is safe to have colonoscopy as early as 3 to 6 months after prostatectomy and/or radiation therapy. But if polyps have to be removed, then there is an increased risk for poor healing and the development of a fistula so it is essential to advise the GI doctor that you have had radiotherapy. It should be obvious to the doctor, though, because radiation usually induces changes in the appearance of the colon that are characteristic for prior radiotherapy.