CHICAGO – Long-term use of estrogen-only therapy increases the risk of postmenopausal women developing breast cancer, according to new data from the Nurses’ Health Study.
The finding, reported April 1 by Dr. Wendy Chen at the annual meeting of the American Association for Cancer Research, suggests that opting for progesterone-free hormone therapy for the treatment of menopause symptoms should not be considered the risk-free alternative to combination progesterone plus estrogen formulations.
Still, the absolute risks were small. During the study’s 1.57 million person-years of follow-up from 1980 through 2008, 5,631 incident invasive breast cancers were diagnosed and 884 of the women died of breast cancer.
To evaluate the breast cancer risk associated with long-term use of both combined and estrogen-only hormone therapy, Dr. Chen of Brigham and Women’s Hospital in Boston and her colleagues evaluated data collected over 28 years through the Nurses’ Health Study. Of the 121,700 study participants who were aged 30-55 years in 1976, women who used estrogen-only therapy for 10.0-14.9 years and for 15.0-19.9 years had, respectively, a 22% and 43% increased risk of developing breast cancer relative to those women not on hormone therapy (HT).
"Our data also confirmed the previously reported increased risk associated with combined hormone therapy," Dr. Chen stated, noting that women in the study who used progesterone plus estrogen for 10-14.9 years had an 88% increased risk of developing breast cancer relative to those not on HT and the risk increased to more than twofold in women using it for up to 20 years. Further, the risk of developing breast cancer does not appear to plateau over time, but rather continues to increase. "The longer women use [HT]. The higher their risk," she said.
In a separate analysis, the investigators restricted the study population to healthy, active postmenopausal women 50-79 years with an intact uterus – the same population observed in the Women’s Health Initiative. "What we saw was a decline in breast cancer risk among women using estrogen-only therapy for less than 10 years, but an increased risk among those women using estrogen for 15-20 years," Dr. Chen said.
Importantly, although breast cancer incidence was increased in the HT groups, "there was no increase in the risk of fatal breast cancer, either with combined or estrogen-only therapy," Dr. Chen stressed. "This is something we are continuing to investigate."
The findings have the potential to increase the confusion regarding HT and to perhaps reignite the hormone therapy debate, Dr. Chen acknowledged in an interview. However, they should be considered in their proper context. "While the relative risk of developing breast cancer in increased, the absolute risk associated with [HT] are low," she said.
The observed effects associated with HT were stronger for estrogen receptor–positive tumors and among thinner women, Dr. Chen noted, adding that the results were similar when limited to women who underwent regular breast cancer screening.
The study was funded by the National Cancer Institute. Dr. Chen disclosed no financial conflicts of interest.