Date issued: April 2020
For review: April 2022
Ref: A-436/MB/Medical Specialities/Family history of bowel (colon) cancer
How common is colon cancer?
The most recent statistics show that approximately 1 in 20 people in the UK will develop cancer of the colon or rectum during their lifetime. It is slightly more common in men. Most colon cancer happens in people over 50, and is commonest over 65.
What causes colon cancer and is it inherited?
Most colon cancers arise "out of the blue" in a family, and are not usually inherited. There is no single cause, and it is likely that there is a mixture of risk factors. These include ‘lifestyle’ or ‘environmental’ factors, as well as mild, genetic influences. Any person who has a relative with colon cancer will have a small increased risk themselves, mainly because they probably share some of these factors.
Only about 15% of colon cancer is strongly inherited. The other 85% of cases result from a combination of adverse risk factors. (When we say a cancer is ‘inherited’ we mean that the tendency or risk of cancer is inherited, not the cancer itself)
Two conditions, called Lynch Syndrome (also known as HNPCC, Hereditary Non Polyposis Colon Cancer) and Familial Adenomatous Polyposis (FAP), cause a high risk of colon cancer; if these are relevant to your family they will have been discussed with you during your appointment.
How can you tell whether a colon cancer is inherited or not?
There is usually not a single test for this. We make an assessment, and consider a number of factors, e.g.:
- Number of people with colon cancer in the family (and the number of people who do not have it).
- Ages that the cancers were diagnosed.
- Any other cancers in the family, especially womb cancer (endometrium) in women.
- Location of the tumour (different parts of the colon are called caecum, sigmoid, rectum etc.)
- More than one colon cancer in the same person.
- Polyps in the colon.
- The precise cell type in the cancer (histology) may be important.
Family history changes over time. You should let your doctor know about any new cancers in the family, because this may change your assessment.
How likely is it that I or my family will develop colon cancer?
Your Geneticist/Gastroenterologist will determine which of the three risk categories applies to you
Low or population risk, the cancer is unlikely to be genetic. Relatives do not have a significantly increased. No special screening tests are necessary.
- Moderate risk, the cancers have arisen because of a combination of adverse factors, including some genetic influences. Close relatives have a moderately increased risk themselves. Colon screening may be worthwhile. A strong genetic cause is not very likely but in some basic genetic screening can be helpful to clarify this.
- High risk, a genetic predisposition to colon cancer is quite likely. There could be a misprint in a gene in the family. We advise colon screening, and we may be able to offer genetic testing.
What about genetic testing?
Genetic testing may be helpful, mainly in families with a significant chance of finding a gene misprint. If this applies to your family, your Gastroenterologist will refer you to Genetics.
We usually have to start any genetic testing with a sample from an affected relative. If we find a gene misprint in the relative with cancer, we can then offer a test to unaffected people in the family.
What about colon screening?
Screening is an important way of reducing the risk of colon cancer. This is usually done by a procedure called colonoscopy. A narrow, flexible tube is inserted into the back passage. This is done with mild sedation, and a general anaesthetic is not necessary. The can look directly at lining of the colon. Cancer often starts from a harmless growth called a "polyp" or "adenoma". Usually, any polyps can usually be removed easily during the colonoscopy. This prevents that particular polyp from developing into a cancer. A colonoscopy carries a small risk of damage, and this needs to be taken into account when deciding whether a person is likely to benefit from colonoscopy.
Is there anything else I can do to reduce the risk?
There is evidence that diet is important in colon cancer risk. People who eat lots of ‘junk’ food, processed food, fat or red meat have a higher risk. It is a good idea to eat plenty of fresh fruit and vegetables, and to reduce the amount of animal fat, particularly red and processed meat. People who are overweight have a higher risk, so do people who smoke or drink excessive alcohol. On the other hand, regular exercise lowers risk and helps to control weight.
Several research studies have shown that aspirin lowers the risk of colon cancer. For example, one study of people with Lynch syndrome, found that the risk was halved in people who took aspirin. At the moment we do not know whether aspirin will be effective in everyone. We do not know the best dose, or how long to take it. At present we cannot be sure that the benefits of taking aspirin outweigh the risk of side effects. It is important that you talk to your GP if you are considering taking aspirin. We expect there will be some national guidance on this in 2020.
What symptoms should I look out for?
Bleeding from the back passage or a change of bowel habit (diarrhoea or constipation persisting for more than a couple of weeks) should be reported to your doctor. Although these symptoms usually have an innocent cause, they may indicate an early colon cancer.