Apparently there is a risk, if a patient has a colonoscopy too soon after radiation for prostate cancer, that the gastroenterologist doing the colonoscopy may not understand what a rectum can look like after prostate radiation. If they decide to biopsy something rather than leave what is obviously radiation damage alone, "horror stories" can result. Another reason I've heard mentioned in favor of having a colonoscopy prior to radiation is colon cancer, if found, takes precedence over prostate cancer.