Bowel cancer (also called colorectal cancer) occurs when abnormal cells in the wall of the large bowel (which includes the colon, rectum and anal canal) grow in an uncontrolled way.
The colon, a muscular tube around 1.5 metres long, absorbs water and nutrients from food. It is divided into four sections – the ascending colon, transverse colon, descending colon, and sigmoid colon. The rectum, which is the lowest six inches of the bowel, is the holding place for stool, which eventually passes out of the body through the anus.
Most bowel cancers are thought to develop from non-malignant growths in the sigmoid colon – the part just above the rectum or on the lining of the wall of the bowel. These non-malignant growths are called adenomas or polyps and they are easily detected by a medical examination. Not all polyps become cancerous.
If a polyp does become cancerous it can lead to the spread of cancer through some or all of the tissue layers making up the colon and rectum, often over several years. The severity of disease is assessed according to how many of the tissue layers the cancer penetrates.
Cancer within the bowel can narrow and block the bowel passage or cause bleeding. In more advanced cases, the cancer can spread beyond the bowel to other organs.
The most common type of bowel cancer is called an adenocarcinoma, which are cancers of the cells lining the colon or rectum’s interior. Other rare types include squamous cell cancers (which start in the skin-like cells of the bowel lining), carcinoid tumours, gastrointestinal stromal tumours, sarcomas and lymphomas.
As most bowel cancers start as polyps, all polyps should be removed to reduce risk of developing bowel cancer. A procedure called a colonoscopy can usually remove most polyps from the bowel. Once removed, the polyp can no longer develop into cancer. Even if a polyp develops into cancer, in its early stages it can be cured by surgery.