Colorectal cancer is a cancer of the colon or rectum. It is sometimes called bowel cancer or cancer of the large intestine. It is one of the most common cancers in the UK. (In contrast, cancer of the small intestine is rare.) Colorectal cancer can affect any part of the colon or rectum, but most commonly first develops in the lower part of the descending colon, the sigmoid colon, or rectum.
In most cases, colorectal cancer develops from a polyp which has formed on the lining of the colon or rectum (see below). Sometimes colorectal cancer begins from a cell within the lining of the colon or rectum which becomes cancerous.
(Some rare types of cancer arise from various other cells in the wall of the colon or rectum. For example, carcinoid, lymphoma, and sarcomas.)
As the cancer cells multiply they form a tumour. The tumour invades deeper into the wall of colon or rectum. Some cells may break off into the lymph channels or bloodstream. The cancer may then spread (metastasise) to lymph nodes nearby or to other areas of the body, most commonly the liver and lungs.
A bowel polyp (adenoma) is a small growth that sometimes forms on the inside lining of the colon or rectum. Most bowel polyps develop in older people. About 1 in 4 people over the age of 50 develop at least one bowel polyp. Polyps are benign (non-cancerous) and usually cause no problems.
However, sometimes a benign polyp can turn cancerous. If one does turn cancerous, the change usually takes place after a number of years. Most colorectal cancers develop from a polyp that has been present for 5-15 years.