What causes cancer?

Information from Cancer Research UK here
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The many causes of cancer

There are about 200 different types of cancer affecting all the different body tissues. What affects one body tissue may not affect another. For example, tobacco smoke that you breathe in may help to cause lung cancer. Over exposing your skin to the sun could give you a melanoma on your leg. But the sun won't give you lung cancer and smoking won't give you melanoma.

Apart from infectious diseases, most illnesses are 'multifactorial'. Cancer is no exception. Multifactorial means that there are many factors involved. In other words, there is no single cause for any one type of cancer.

Carcinogens

A 'carcinogen' is something that can help to cause cancer. Tobacco smoke is a powerful carcinogen. But not everyone who smokes gets lung cancer. So there must be other factors at work.

Age
Most types of cancer become more common as we get older. This is because the changes that cause a cell to become cancerous in the first place take a long time to develop. There have to be a number of changes to the genes within a cell before it turns into a cancer cell. The changes can happen by accident when the cell is dividing. Or they can happen because the cell has been damaged by carcinogens and the damage is then passed on to future 'daughter' cells when that cell divides. The longer we live, the more time there is for us to accumulate these genetic mistakes in our cells.

Genetic make up

There have to be a number of genetic mutations within a cell before it becomes cancerous. Sometimes we are born with one of these mutations already. This does not mean we will get cancer. But with one mutation from the outset, it makes it more likely statistically that we will. Doctors call this 'genetic predisposition'.

The BRCA1 and BRCA2 breast cancer genes are examples of genetic predisposition. Women who carry one of these faulty genes have a higher chance of developing breast cancer than women who do not.

The BRCA genes are good examples for another reason. Most women with breast cancer do not have a mutated BRCA1 or BRCA 2 gene. Less than 5% of all breast cancer is due to these genes. So although women with one of these genes are individually more likely to get breast cancer, most breast cancer is not caused by a high risk inherited gene fault.

This is true of other common cancers where some people have a genetic predisposition, for example colon (large bowel) cancer.

Researchers are looking at the genes of people with cancer in a study called SEARCH. They also hope to find out more about how other factors might interact with genes to increase the risk of cancer. Information about this study is on our clinical trials database. Please note that you cannot volunteer for this study.

The immune system

People who have problems with their immune systems are more likely to get some forms of cancer. This group includes people who

• Have had organ transplants and take drugs to suppress their immune systems to stop organ rejection
• Have AIDS
• Are born with rare medical syndromes which affect their immunity

The kinds of extra cancers that affect these groups of people fall into two, overlapping groups

• Cancers that are caused by viruses, such as cervical cancer or some lymphomas
• Lymphomas

Chronic infections or transplanted organs can continually stimulate cells to divide. This continual cell division means that immune cells are more likely to acquire mutations and develop into lymphomas.

Diet

Cancer experts estimate that changes to our diet could prevent about one in three cancer deaths in the UK. In the western world, many of us eat too many animal fats and not enough fresh fruit and vegetables. This type of diet is known to increase your risk of cancer. But how exactly we should alter our diets is not clear. There is more about this in the page on diet causing cancer.

Sometimes foods or food additives are blamed for directly causing cancer and described as 'carcinogenic'. This is often a distortion of the truth. Sometimes a food is found to contain a substance that can cause cancer but in such small amounts that we could never eat enough of it to do any harm. And some additives may actually protect us. There is more about food additives in the page on diet causing cancer.

Day to day environment
By this we mean what is around you each day that may help to cause cancer. This could include

• Tobacco smoke
• The sun
• Natural and man made radiation
• Work place hazards
• Asbestos

Some of these are avoidable and some aren't. Most are only contributing factors to causing cancers - part of the jigsaw puzzle that scientists are still trying to put together. There is more about this in the page on causes of cancer in the environment.

Viruses

Viruses can help to cause some cancers. But this does not mean that these cancers can be caught like an infection. What happens is that the virus can cause genetic changes in cells that make them more likely to become cancerous.

These cancers and viruses are linked

• Cervical cancer and the genital wart virus, HPV
• Primary liver cancer and the Hepatitis B virus
• T cell leukaemia in adults and the Human T cell leukaemia virus

There will be people with primary liver cancer and with T cell leukaemia who haven't had the related virus. But infection may increase their risk of getting that particular cancer. With cervical cancer, scientists now believe that everyone with an invasive cervical cancer will have had an HPV infection beforehand.

Many people can be infected with a cancer-causing virus, and never get cancer. The virus only causes cancer in certain situations. Many women get a high risk HPV infection, but never develop cervical cancer. Another example is Epstein-Barr virus (EBV). These are some facts about this common virus

• Most people are infected with EBV
• People who catch it late get glandular fever but this does not cause cancer
• In sub-Saharan Africa, EBV infection and repeated attacks of malaria together cause a cancer called Burkitt's lymphoma that affects children
• In China, EBV infection (together with other unknown factors) causes naso-pharyngeal cancer
• In AIDs patients and transplant patients EBV can cause lymphoma
• In the UK, about 4 out of 10 cases of Hodgkin's disease seem to be related to EBV infection