Doctors should ensure they ask female patients if they have an identical twin sister whenever breast cancer is diagnosed, say scientists.

New research shows women who have an identical twin sister with the disease are four times more likely than average to go on to develop it themselves.

This is higher than was previously thought - even taking account of the fact that identical twins are an exact genetic match.

Doctors could use this information to identify women who are particularly susceptible to breast cancer and advise them accordingly

Professor Thomas Mack

The researchers, based in the University of Southern California, studied 2,562 pairs of identical and non-identical twins with either one or two cases of breast cancer and compared their rates of the disease with the general population.

Previous research had shown that, before the menopause, where one identical twin developed breast cancer their sister had a dramatically increased risk of doing likewise.

The new paper shows that even after the menopause the risk is increased.

And not only does risk increase, but the onset of breast cancer occurs much earlier.

An identical twin has a dramatically higher chance of developing breast cancer within five years of her sister than a non-identical twin or sister.

Screening

Lead researcher Professor Thomas Mack said: "Studying twins is a useful way to compare genetics and the environment.

"The fact that non-identical twins have the same risk as a mother or sister despite having a more similar upbringing can show us to what extent genetics play a part in the development of breast cancer.

"Doctors could use this information to identify women who are particularly susceptible to breast cancer and advise them accordingly."

More regular screening and advice about prevention can be offered to high risk women.

The researchers say one way of easily identifying them would be to ask every patient diagnosed whether they are a twin.

As knowledge of genetic and environmental risk factors grows this practice will become more and more useful in prevention and early detection of cancer and other diseases.

Sir Paul Nurse, interim chief executive at Cancer Research UK said: "We know that having a close family member with breast cancer increases a woman's risk, and this is raised dramatically when that relative is an identical twin.

"It makes sense to keep a more watchful eye over women who may be more susceptible to the disease."

A spokeswoman for the charity Breakthrough Breast Cancer said: "This study highlights the need for more research into the genetics of breast cancer.

"Doctors need to be aware that some women may be at a higher risk of developing the disease. Women need to be advised accordingly so they can make informed decisions about the management of their risk."

The research is published in the British Journal of Cancer.