Light micrograph section through human tissue showing rectal adenocarcinoma. Adenocarcinomas of the rectum and colon make up 95 percent of all colorectal cancer cases. In the gastrointestinal tract, rectal and colon adenocarcinomas develop in the cells of the lining inside the large intestine. These cancers typically start as a growth of tissue called a polyp. A particular type of polyp called an adenoma may at some stage develop into cancer.