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Smoking more than doubles the risk of developing breast cancer among women with a strong family history of the disease, a study has found.
The major international review is the first to show a connection between the disease and tobacco in women with the highest risk breast cancer genes, BRCA1 or BRCA2.
About half of women with faulty versions of these genes will develop cancer by the age of 70.
The research has found the risk is as low as 35 per cent among non-smokers, rising to almost double, 65 per cent, among smokers with a mutation.
"This study suggests that women with faulty genes can effectively halve their risk of developing breast cancer by not smoking," said University of Melbourne researcher Dr Mark Jenkins, who was part of the international collaboration.
"If you have a faulty breast cancer gene or a strong family history of breast cancer, not smoking may reduce your risk of developing breast cancer, and you will also get all the other known health benefits of not smoking."
The review, published in the journal Breast Cancer Research and Treatment, analysed data collected from 780 women involved in large breast cancer trials.
All the women were aged under 50 and had a faulty BRCA gene, and half had developed cancer.
Analysis found that the women who developed breast cancer were more likely to have smoked than those who were cancer free, Dr Jenkins said.
Women with a faulty BRCA1 gene had a 2.3 increased risk after smoking for five years or more, and BRCA2 smokers had a 2.6 increased risk.
The risk increased the longer a woman smoked, rising about 7 per cent every year they maintained the habit.
Dr Jenkins says BRCA1 and BRCA2 genes, when functioning normally, repair the type of DNA damage caused by carcinogens such as cigarette smoke.
Women who inherit a faulty copy of the genes are less able to repair DNA damage. The fact that not all women with faulty genes will develop breast cancer suggests that environmental factors influence their cancer risk.
"This study suggests that smoking plays a major role," he said.
About one in 300 women have a faulty BRCA1 or BRCA2 gene. Of those, about between 20 and 40 per cent will develop breast cancer by the age of 50, rising to 60 per cent by the age of 70.
Some high-risk women choose to have mastectomies, their ovaries removed, or preventative chemotherapy.
- AAP
