Risk varied according to BMI, with greater risks for thinner women.
Longer duration of hormone therapy increased breast cancer risk.
PHILADELPHIA - A new analysis of the California Teachers Study, which analyzed hormone replacement therapy use among 2,857 women for almost 10 years, underscores the need for personalized risk-benefit discussions before women begin hormone therapy.
"This is evidence that the story is complicated," said Tanmai Saxena, an M.D./Ph.D. student at the Keck School of Medicine at the University of Southern California. "The benefits of hormone therapy for relief of postmenopausal symptoms among women are clear, but the risks are more complicated than we had previously thought."
In a report published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research, Saxena and colleagues found that compared with women who had never used hormone therapy, women who used estrogen therapy for more than 15 years had a 19 percent greater risk of developing breast cancer.
Women who used combined therapy with estrogen plus progestin for 15 or more years had an 83 percent greater risk. Breast cancer risk was highest among women who used the combination regimen.
Breast cancer risk seemed dependent on body mass index (BMI). Those with a BMI less than 30 appeared to have an increased risk of breast cancer with combined hormone therapy; the risk was strongest among women with BMI less than 25. In contrast, obese women (i.e., BMI of 30 or more) had no further increase in risk associated with using combined hormone therapy.
Finally, the risk of breast cancer was confined to tumors that were positive for both estrogen and progestin receptors. The risk was somewhat weaker for HER2 negative tumors.
Susan Hankinson, Sc.D., professor of medicine at Harvard Medical School, said the findings underscore the reality that even following the Womens Health Initiative (WHI) trial and large prospective studies including the California Teachers Study, there are still questions that remain.
"These results add new evidence that risk does vary by other personal characteristics. However, for now, the public health message remains essentially the same. There is an increased risk of breast cancer from hormone use, and further studies will address the question of how specific that risk is," said Hankinson, who is a senior editor of Cancer Epidemiology, Biomarkers & Prevention.
Follow the AACR on Twitter: @AACR
Follow the AACR on Facebook: facebook.com/aacr.org