May 9, 2006 — Long-term use of unopposed estrogen is associated with an increased risk for invasive breast cancer, especially for the development of estrogen receptor positive (ER+) and progesterone receptor positive (PR+) tumors, according to an analysis of data from the Nurses' Health Study.
Wendy Y. Chen, MD, MPH, with the Brigham and Women's Hospital and Harvard Medical School, in Boston, Mass, reported their findings in the May 8, 2006, issue of Archives of Internal Medicine.
A total of 28 835 women were included in the final follow-up period, including 11 508 postmenopausal women who had a hysterectomy and reported information on estrogen use at baseline in 1980. Data on estrogen use were collected through a questionnaire given every 2 years.
A total of 934 invasive breast cancers were diagnosed in the study population. Breast cancer risk increased with duration of unopposed estrogen use among longer-term users; the highest risk was seen for the development of ER+ and PR+ tumors.
Overall, the risk for invasive breast cancer increased with increasing duration of use (P for trend < .001). The respective risks for unopposed estrogen use of less than 5 years, 5 to 9.9 years, 10 to 14.9 years, 15 to 19.9 years, and 20 years or longer were 0.96 (95% confidence interval [CI], 0.75 - 1.22), 0.90 (95% CI, 0.73 - 1.12), 1.06 (95% CI, 0.87 - 1.30), 1.18 (95% CI, 0.95 - 1.48), and 1.42 (95% CI, 1.13 - 1.77).
For the development of ER+/PR+ breast cancers, the risk was noted to be significant after 15 years of use (relative risk, 1.48; 95% CI, 1.05 - 2.07).
"Among women who used unopposed estrogen therapy for less than 20 years, we did not observe a statistically significant increased risk of breast cancer overall. However, breast cancer risk was significantly elevated with longer durations of use, primarily for ER+/PR+ cancers," which is consistent with other study findings, the researchers conclude
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