Bowel cancer

Bowel cancer overview

The human colon is a muscular, tube-shaped organ measuring about 1 metre long. It extends from the end of your small bowel to your anus, twisting and turning through your abdomen (belly). The colon has three main functions. 

• To digest and absorb nutrients from food
• To concentrate faecal material by absorbing fluid (and electrolytes) from it
• To store and control evacuation of faecal material.

The right side of your colon plays a major role in absorbing water and electrolytes, while the left side is responsible for storage and evacuation of stool. 

Cancer is the transformation of normal cells. The transformed cells grow and multiply abnormally.

• Left untreated, these cancers grow and eventually spread through the colon wall to involve the adjacent lymph nodes and organs. Ultimately, they spread to distant organs such as the liver, lungs, brain and bones.
• Cancers are dangerous because of their unbridled growth. They overwhelm healthy cells, tissues and organs by taking their oxygen, nutrients and space.
• Most bowel cancers are adenocarcinomas - tumours that develop from the glands lining the colon’s inner wall.
• These tumours are sometimes referred to as colorectal cancer, reflecting the fact that the rectum, the end portion of the colon, can also be affected.

About 38,000 people are diagnosed with colorectal cancer in the UK each year. 

Excluding non- melanoma skin cancer, colorectal cancer is the 3rd commonest cancer overall. It is the 2nd commonest affecting women (after breast cancer), and the third commonest in men (after prostate and lung cancer). About two-thirds of colorectal cancers occur in the large bowel (colon), and one-third arise in the rectum.

Bowel cancer causes

Most colorectal cancers arise from adenomatous polyps - clusters of abnormal cells in the glands covering the inner wall of the colon. Over time, these abnormal growths enlarge and ultimately degenerate to become adenocarcinomas.

People with any of several conditions known as adenomatous polyposis syndromes have a greater-than-normal risk of colorectal cancer.

• In these conditions, numerous adenomatous polyps develop in the colon, ultimately leading to colon cancer.
• The cancer usually occurs before age 40.
• Adenomatous polyposis syndromes tend to run in families. Such cases are referred to as familial adenomatous polyposis (FAP).

Another group of colon cancer syndromes, termed hereditary non-polyposis colorectal cancer (HNPCC) syndromes, also run in families. In these syndromes, colon cancer develops without the precursor polyps.

• FAP and HNPCC syndromes are associated with a genetic abnormality. This abnormality has been identified, and a test is available. People at risk can be identified through genetic screening.
• Once identified as carriers of the abnormal gene, these people require counselling and regular screening to detect precancerous and cancerous tumours.
• HNPCC syndromes are sometimes linked to tumours in other parts of the body.