Bowel, or colorectal, occurs in the colon (large intestine) or back passage (rectum). About 80% of bowel cancers occur in the large intestine and 20% in the rectum. Anal cancer (cancer of the anus) is rare.

Our body is made up of more than 37 trillion cells that change and grow naturally. Cancer starts when cell changes go wrong and cells begin to divide in a uncontrolled way. Bowel cancer begins as a solid tumour, usually developing from small polyps which are found on the bowel wall. Many of us have these polyps, and as we age they are more common. Most will never turn into cancer, but over time they can. 

Your doctor can remove polyps to stop them turning into cancer. In common with many other cancers, bowel cancer is most worrying when it has spread (metastasised). At the beginning it goes through the wall of the bowel, then into lymph nodes and finally to distant organs – most often the liver.

Who gets bowel cancer?

More than 41,000 people annually are diagnosed with bowel cancer and it is the 2nd most prevalent cancer that affects both men and women. 1 in 14 men (7%) and 1 in 19 women (5%) will be diagnosed with bowel cancer in their lifetime. Bowel cancer can affect people of any age, although 94% of bowel cancer is diagnosed in those over 50 and 4 out of ten people are diagnosed over the age of 75.

Stages in the development of bowel cancer

There are three categories which Doctors use to define bowel cancer; Dukes' system, the number system and the TNM system. Bowel cancer develops as a solid tumour and through staging it doctors can define how large the tumour is, whether it has invaded lymph nodes and if it has spread (metastasised) to other organs – most commonly the lungs or the liver.

Dukes' system
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Letter Meaning
A The cancer is no further than the inner lining of the bowel.
B The cancer has spread into the muscle layer of the bowel.
C The cancer has spread to a lymph node.
D The cancer has spread to other organs, such as the liver.

Number system
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Number Definition
0 Contained cancer cells are identified in the lining of the bowel. This is a very early stage of bowel cancer that is more treatable. There is little risk of cancer cells having spread at this stage.
1 The cancer has grown through the inner lining of the bowel only.
2 The cancer is localised and there is no cancer in the lymph nodes (lymph nodes are special filters that can trap cancer cells or bacteria that are traveling around the body. Lymph nodes are key to the immune response).
3 A B C is divided into 3A, 3B and 3C. Stage 3A = the cancer remains in the inner layer of the bowel wall. It may have also grown slightly into the muscle. A few lymph nodes may contain cancer cells. Stage 3B = the cancer has grown into the outer lining of the bowel wall. A few lymph nodes may contain cancer cells. Stage 3C = the cancer has spread to 4 or more lymph nodes. The cancer has not spread further to another part of the body.
4 The cancer has spread to organs such as the liver ,and is found in many lymph nodes.

TNM  system
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Name Number Meaning
Tumour 1-4 1 is the smallest tumour to 4 being the largest
Node 0-3 0 means no cancer in the nodes to 3 where cancer is located in a number of nodes
Metastasis 0-1 0 means no cancer spread to 1 where spread is identified