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Breast cancer risk in twins

What is the risk to an identical twin sister of a patient with fast growing breast cancer?

Cancer Research UK research has suggested that identical twins of women already diagnosed with breast cancer have a 1 in 3 risk of getting breast cancer themselves at some point in their lives. The risk for women generally is 1 in 8 over the whole lifetime. So, according to this research, identical twin sisters of diagnosed breast cancer patients have around 3 times the risk of the average woman. Research from America has estimated the increase in risk as 4 times the risk of the average woman. 

It is important to remember that the numbers above are just statistics and cannot predict what will happen in your individual case. As an identical twin sister of someone with breast cancer, you have a higher than average risk of getting the disease yourself. You can speak to your doctor about having regular breast checks.

Identical twin sisters are genetically the same. Breast cancer is a disease that can have a lot of contributory causes. The genetic make up is important. But environmental factors could be important too. All identical twins have shared their mother's womb. Most will have grown up together and shared the same environment, eaten the same type of diet and so on. The important thing about both these pieces of research is that there wasn't the same increase in risk in non identical twins. They will usually also have shared the same environment. The new findings indicate that genetic factors may be very important in breast cancer.

Generally, doctors believe that about 3 out of every 100 breast cancers (3%) are due to an inherited faulty gene. The other 97 out of 100 breast cancers (97%) are thought to be sporadic, which means they can happen to anyone and cannot be predicted. The breast cancer genes BRCA1 and BRCA2 are known to greatly increase the risk of breast cancer. But these genes only account for about 2 out of every 100 breast cancers diagnosed (2%). Inherited risk is probably more often caused by a number of different genes with a smaller effect working together.

If you have a significant family history of breast cancer on one side of your family, there may be one or more inherited faulty genes at work. This is more likely to be the case if the breast cancers were diagnosed at a young age (under the age of 50). There is more detail about what doctors mean by significant family history in the main breast cancer section.

If you are the identical twin of a woman diagnosed with breast cancer, talk to your doctor. You may want to ask your sister's breast cancer specialist to give you advice on what you should do. Depending on your family history, you may be able to have testing for a known breast cancer gene. Although, as we have said, it is likely that there are a number of genes involved in causing breast cancer and we can't test for all of those yet. There are regional family cancer clinics all over the UK who can advise on these matters and on screening. You can get a referral from a specialist breast clinic or, in some parts of the country, from your GP.

There is information about genes and breast cancer in the main breast cancer section.

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Updated: 1 May 2013