Can an overdose of ionized radiation in one year cause colon polyps more than 20 years later?

No. Overdose of radiation can cause all kinds of problem that are very serious but polyps is not one of them. Polyps however are very common and there are even genetic predisposition to them such as familial polyposis syndrome. Colonoscopy has become a standard of care and when polyps are encountered they are removed and more frequent colonoscopy performed.
Colonic polyps. Most of the colonic polyps are benign but some have the potential to become cancerous. It is not possible to tell by just looking and thus polyps are removed to be examined microscopically to be certain that they are no early cancerous changes. There are some genetically related syndromes that predispose young patients to develop multiple polyps with malignant potential but radiation is not relate.