Scientists have discovered that large bowel cancer develops because of defects in the genes of cells lining the bowel. These cells start to multiply and form a small protrusion or ‘polyp’ on the bowel surface. The majority of polyps remain localised and cause no symptoms.

However, further changes can occur in the cells within a polyp, which cause them to become cancerous. This is why removal of polyps can prevent cancer.

Cancer develops when cells begin to multiply at an abnormal rate. Normally, cells die and are replaced in equal measure. When cells begin to multiply at a faster rate than they should, a growth forms of all the unwanted cells. This can go on to form a cancerous growth. The characteristics of cancer cells are that they invade the surrounding normal tissue and may spread to other organs. The reason for the altered behaviour of cells is linked to an abnormality in their genes. It is known that a number of specific gene abnormalities (or “mutations”) play an important part in cancer development and spread.

People can be born with these gene mutations, in which case other family members may also have an increased risk of several types of cancer, or these genetic abnormalities can arise during a person’s lifetime.

Depending on which genes are affected, in a very few patients this may make cancer inevitable. In most people, a single gene abnormality does not cause any problems unless other genetic abnormalities arise. As a consequence of these accumulated genetic abnormalities either the destruction of abnormal cancer cells by the body’s natural defences is prevented or rapid, uncontrolled growth of cancerous cells starts.

The causes of the genetic defects which develop during a person’s lifetime and have not been inherited are unknown, although some of these gene defects probably originate because of our diet. Patients with some long-standing inflammatory diseases of the bowel, such as Crohn’s disease or ulcerative colitis, may also have an increased risk of developing bowel cancer.

A diet rich in fresh vegetables and fruit and plenty of fibre seems to help protect against bowel cancer and there is some evidence that a diet containing much meat may increase the risk. There is also evidence that patients who regularly take anti-inflammatory drugs, such as aspirin, may be at a lower risk of developing cancer, but at the moment it is felt that the potential risks of taking aspirin regularly outweigh the benefits.

Often a bowel cancer causes no symptoms at the beginning. It may bleed onto the surface of the motion (stool) or cause changes in bowel habit, such as unusual episodes of diarrhoea or constipation or an increased amount of mucus in the stool. A cancer can cause a partial or complete blockage of the bowel leading to abdominal pain, windy distension (bloating) and, in severe cases, vomiting.

If small amounts of bleeding go on for some time, anaemia may cause tiredness and decreased ability to work and exercise.

Weight loss is usually a late symptom. Sometimes a cancer can perforate a hole through the bowel wall, so that bowel contents leak into the abdomen. This causes severe pain and the need for urgent surgery.

One diagnostic test is an X-ray examination using barium to outline the bowel (barium enema). A small tube is placed in the anus and the liquid barium and some air are introduced, with the patient on the x-ray table. The barium outlines the bowel and X-rays are taken to show any irregularity in the bowel wall caused by the cancer.

Secondly, an examination can be made with a flexible telescope passed up from the anus. A sigmoidoscope can examine the lower bowel, a colonoscope is longer and can examine the whole of the large bowel. If any abnormality is seen, a small sample (biopsy) can be taken for analysis.

To help decide precise treatment it may be necessary to see the extent of the cancer and so a scan may be arranged.

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When polyps are found they can often be removed using a colonoscope. A wire ‘snare’ is manoeuvred around the base of the polyp, tightened, and the polyp is separated from the bowel wall by passing a small electric current through the wire.

After removal of a polyp, it will be examined using a microscope. Usually the polyp is made up of abnormal cells, but these are not cancerous. Sometimes an area of cancer is found within a polyp. If the cancer is confined to the polyp its removal is curative. If the examination suggests there is a risk that the cancer cells are not completely removed, a second colonoscopy or an operation to remove that part of the bowel will be advised.

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By the time of diagnosis, most cancers are situated within the bowel wall and there may be no evidence of the original polyp. Such cancers require an operation for their removal, but the type of operation will vary depending on where the cancer is. Sometimes it is not possible to join the bowel back together and so an opening (stoma) onto the skin of the abdomen may have to be made. A changeable bag will cover the opening to collect the stool. The opening is called an ‘ileostomy’ or a ‘colostomy’ depending on which part of the bowel is used to make it. Nowadays it is rarely necessary for such a stoma to be permanent. If it is temporary, it will be closed at a second operation after recovery from the initial surgery.

After removal of a cancer it is examined to decide what risk there is of recurrence because of the spread of cancer cells before the operation. Some patients may benefit from chemotherapy or radiotherapy. A number of drugs are available, or are being tested, to reduce the risk of recurrence or to treat a cancer if it recurs. The surgeon may ask another specialist (an oncologist) to advise on drug treatment.

A person who has developed one or more polyp(s) may develop others years later. Another colonoscopy may therefore be advised after an interval. It is also known that patients who have had a bowel cancer have an increased risk of developing another. Some surgeons will routinely check the bowel with colonoscopy a year or more after an operation for removal of a cancer.