For the study, which was based on data from the Women's Health Initiative, Phipps and colleagues analyzed the detailed reproductive histories of some 150,000 postmenopausal women, more than 300 of whom went on to develop triple-negative breast cancer. "This particular study is significant because it is one of the largest studies ever conducted on the impact of reproductive history on triple-negative breast cancer," Phipps said.

Triple-negative breast cancer, which refers to any breast cancer that does not express the genes for estrogen receptor (ER), progesterone receptor (PR) or Her2/neu, accounts for only 10 percent to 20 percent of all breast cancers, and only in the past decade have researchers become aware that this cancer subtype exists. "This research reinforces the notion that breast cancer is not just one disease," Phipps said.

"The mechanisms that lead to triple-negative breast cancer are likely different from those that lead to other forms of the disease. We still have a lot to learn about what causes this more aggressive form of breast cancer, but we hope that research like this will help us develop better tools to identify those women at greatest risk."

It is known that this cancer subtype is more predominant in African American women and it tends to be diagnosed at an earlier age. Researchers also know there is a strong link between genetic mutations in the so-called "breast cancer gene," BRCA1, and triple-negative breast cancer.

"More research is needed to better understand the causes of the most aggressive and lethal forms of breast cancer. While this study adds to our knowledge base, it should not change women's approaches to breast cancer screening," Phipps said.

Source: Fred Hutchinson Cancer Research Center