What percentage of rectal polyps become cancerous?

About 10%: It was felt that malignancies of bowel i.e. rectal polyps accumulate mutations to go from adenomatous polyps to malignant polyps to cancer it now appears that about 10% of such lesions will progress to cancer. In bowel adjacent to malignancy, most normal appearing mucosal cells produce cancer protein having undergone genotypic changes. These cells eventually lead to new Ca's. in bowel.