In the UK, approximately 36,000 people each year develop cancer of the bowel. This makes it a very common type of cancer.

The cause for the great majority of bowel cancers remains unknown but it is commoner in some families than others. This may be due to shared environmental factors within families (such as diet) but may also be due to an inherited (genetic) factor which increases the risk of developing bowel cancer. However, no specific gene has, so far, been identified as the cause of this increased susceptibility.

In addition, there are two rare inherited conditions which considerably increase the risk of developing bowel cancer: familial adenomatous polyposis (FAP) and hereditary non polyposis colorectal cancer (HNPCC). About 1 in 100 people with bowel cancer will have FAP and perhaps up to 1 in 50 will have HNPCC. People who have either of these conditions can develop bowel cancer at a much younger age than normal (in their 40s to 50s rather than 60s to 70s).

If your doctors find you have a very strong family history of bowel cancer, you might be offered genetic testing. This is optional and you don’t have to have it. If you decide to be tested, it is possible (and quite likely) that the test won’t find any genetic abnormalities. This could be because the test failed to pick up any abnormalities or because there might be a gene involved that we don’t yet know about.

Even if you don't have genetic testing or you have the test but no mutation is found, doctors may still think that your relatives are at an increased risk of bowel or endometrial cancer. This will be based on their view of your family history, or because you have been found to have FAP or HNPCC.

Because medical consultations are always confidential, no-one can tell your family about this apart from yourself. The doctor will encourage you to share your information so that your relatives can get counselling and bowel cancer screening. Doctors are likely to advise counselling and a programme of screening for your close relatives. Decisions on the type of screening to be used, how often it should be done and which family members should be screened are worked out on an individual basis depending on the degree of risk.

The two main screening methods for bowel cancer are the faecal occult blood test and colonoscopy.

• The faecal occult blood tests involves a simple chemical test on the stools to detect blood. If the test is positive it does not necessarily mean there is a cancer. But a positive result will mean other tests are needed to find out whether or not there is a tumour.
• Colonoscopy involves inserting a thin flexible tube with a camera on the end into the bowel through the back passage.Doctors can look at the whole of the bowel and check for anything abnormal. Although the test is sometimes uncomfortable, it is not painful and can usually be done as an out-patient.

The aim of these tests is to discover pre-cancerous changes or very early cancers which can be treated with relatively minor surgery. They do not always prevent the cancers from forming in the first place.

A further approach being tested in clinical trials for people with FAP or HNPCC is to take regular aspirin together with a special form of starch, which is difficult to digest, in the hope that this might actually prevent tumours from forming in the bowel. It will be some years before we will know any results from these studies. If you have a gene mutation you might be invited to join this trial. There is no obligation to join the trial. It is up to you.

There are great variations in risk in different families. This means that decisions on the type of screening to be used, frequency of screening, and which members of a family should be screened are worked out on an individual basis and will vary from family to family.