Breast cancer is the leading cause of death in women with cancer worldwide.
Researchers have identified a molecular marker in normal breast tissue that can predict a woman's risk for developing breast cancer, the leading cause of death in women with cancer worldwide.
The study builds on previous research finding that women already identified as having a high risk of developing cancer namely those with the BRCA1 or BRCA2 mutation, or women who did not give birth in before their 30s - had a higher number of mammary gland progenitor cells.
In the new study, researchers from Harvard University examined biopsies, some taken as many as four decades ago, from 302 women in the Nurses' Health Study and the Nurses' Health Study II, who had been diagnosed with benign breast disease.They compared tissue from the 69 women who later developed cancer to the tissue from the 233 women who did not.
AT A GLANCE
Researchers found that the women were five times as likely to develop cancer if they had a higher percentage of Ki67, a molecular marker that identifies proliferating cells, in the cells that line the mammary ducts and milk-producing lobules.
These cells, called the mammary epithelium, undergo drastic changes throughout a woman's life, and the majority of breast cancers originate in these tissues. Doctors already test breast tumours for Ki67 levels, which can inform decisions about treatment, but this is the first time scientists have been able to link Ki67 to precancerous tissue and use it as a predictive tool.
"Instead of only telling women that they do not have cancer, we could test the biopsies and tell women if they were at high risk or low risk for developing breast cancer in the future," said Kornelia Polyak from Harvard University.
"Currently, we are not able to do a very good job at distinguishing women at high and low risk of breast cancer. By identifying women at high risk of breast cancer, we can better develop individualised screening, and also target risk-reducing strategies," said Rulla Tamimi of Harvard University.
To date, mammograms are the best tool for the early detection, but there are risks associated with screening; false positive results, false negative results, and overdiagnosis could cause psychological distress, delay treatment, or lead to overtreatment, according to the National Cancer Institute (NCI).
Mammography machines also use low doses of radiation. While a single mammogram is unlikely to cause harm, repeated screening can potentially cause cancer, though the NCI says that the benefits "nearly always outweigh the risks." "If we can minimise unnecessary radiation for women at low risk that would be good," said Tamimi.
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 "The study of normal cell division in a breast (left) versus the mutation BRCA1 or BRCA2 (right) has expanded as Harvard research")
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