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The research found that the highest rates of breast cancer in women with a faulty BRCA1 gene were seen between the ages of 41-50. This peak occurred a decade later for those with a faulty BRCA2 gene.
Rates of ovarian cancer in women with either faulty gene were highest between the ages 61-70.
The researchers believe the analysis could help provide more information for women with a family history of the diseases to help make decisions around ways to reduce their risk.
The Cancer Research UK-funded scientists at the University of Cambridge followed nearly 10,000 women with faults in the BRCA1 or BRCA2 genes. These faults can be inherited and are often found in women with a family history of breast and ovarian cancers.
Professor Arnie Purushotham, Cancer Research UK's senior clinical adviser, said: “Women who carry faulty BRCA genes are much more likely to develop breast or ovarian cancers, and this large study could help women, and their doctors, better understand their risk of developing these cancers.”
At the start of the study 5,046 of the women were free from cancer and 4,810 had breast or ovarian cancer, or both. During an average follow-up of 5 years, 426 women were diagnosed with breast cancer, 109 with ovarian cancer, and 245 with breast cancer in the breast opposite to the one in which cancer was previously diagnosed (contralateral breast cancer).
The researchers looked at the number of these cancers diagnosed across 10-year age groups over the course of the follow up period.
The study, which is published in the Journal of the American Medical Association, estimates that around 7 in 10 women with faults in either BRCA gene will develop breast cancer by the age of 80.
And after a first case of breast cancer, the risk of developing a tumour in the opposite breast within 20 years of diagnosis was 4 in 10 for women with BRCA1 faults and more than 2 in 10 for women with BRCA2 faults.
They also estimate that more than 4 in 10 women with faults in BRCA1, and less than 2 in 10 women with BRCA2 faults, will develop ovarian cancer by age 80.
“This information could help women decide the steps that they may want to take to reduce their risk of breast cancer, such as preventative surgery, medication or lifestyle changes,” added Purushotham.
The breast cancer risk for women with either gene fault increased with the number of close relatives who also had the disease. Women with 2 or more such relatives had double the risk compared to those with no family history.
“We have been able to provide the most precise estimates of age-specific risks to date,” said lead author Dr Antonis Antoniou. “These should provide more confidence in the counselling and clinical management of women with faults in the BRCA1 and BRCA2 genes.”
More than 55,000 cases of breast cancer are diagnosed each year in the UK. But fewer than 2 out of every 100 breast cancers are caused by an inherited faulty gene.
Only women with a strong family history of breast or ovarian cancer are tested for faults in the BRCA genes in the UK.
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