Women who have a strong family history of breast cancer, but who do not have the faulty BRCA gene, are four times more likely to develop the disease, a Canadian study has found.
The findings suggest that a significant family history is enough to warrant starting preventive treatment, such as tamoxifen.
Currently, it is known that carriers of the BRCA gene are up to seven times more likely to develop breast cancer than women who do not have the gene mutation.
However, the risk of disease for women with a family history of breast cancer has never been quantified.
For this latest study, researchers selected 1,492 women from 365 families. All of the families had tested negative for BRCA1 and BRCA2 mutations.
Each family contained two or more breast cancer diagnoses under the age of 50 or three cases diagnosed at any age.
Family history was updated throughout the study using telephone interviews.
After an average follow-up of six years, a total of 65 women were found to have developed cancer, compared with an expected number of 15 calculated for a population without a family history of breast cancer.
Women with a family history of breast cancer were 4.3 times more likely to develop the disease than women who had no family history.
Lead researcher Dr Steven Narod, from the University of Toronto, said: 'This is the first time the breast cancer risk for this group of women has been measured, and it's significantly higher than that of the general population.
'It's important to think about action to prevent breast cancer in women who are at higher risk of developing the disease.'
Dr Lesley Walker, director of cancer information at Cancer Research UK, said: 'The findings will help to strengthen advice to women with a strong family history of breast cancer but no evidence of BRCA mutation.'
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