While many studies have looked at the link, the new research sheds some new light on the topic, says researcher Tanmai Saxena, an MD/PHD student at the University of Southern California Keck School of Medicine in Los Angeles.
"The study further develops which women are at particular risk with HRT," he tells WebMD. Among his findings: "There is a risk associated with long-term HRT, both estrogen and combination [therapy]. As it turns out, the women who are thinner at menopause when they go on HRT have a higher risk of breast cancer."
The new analysis included 2,857 women participants in the California Teachers Study, all diagnosed with invasive breast cancer and followed on average for about 10 years.
The study is published in Cancer Epidemiology, Biomarkers & Prevention.
Saxena and his colleagues looked at the type of hormone therapy used -- estrogen alone or combined with progestin -- and for how long; the researchers took into account the women's body mass index (BMI) and other factors.
When the researchers compared women who had never used hormone therapy to those who reported 15 or more years of estrogen therapy, the estrogen users had a 19% greater breast cancer risk.
Those who used combination therapy for 15 or more years fared worse, with an 83% increased breast cancer risk.
Why so high for the combination therapy? Progestin signals breast tissue to divide, Saxena says, and rapid cell replication boosts the risk of getting cancer.
Body mass index played in to the degree of risk, Saxena found. Women with a BMI of 30 or above, considered obese, did not have an elevated breastcancer risk with increased duration of hormone use, but those with BMI below 29.9 did. However, Saxena points out, obesity alone is a risk factor for breast cancer.