- Estrogen is associated with reduced breast cancer incidence in postmenopausal women undergoing hysterectomy, but progestin+estrogen is associated with an increased incidence in postmenopausal women with an intact uterus.
- Long-term data from 2 randomized, placebo-controlled trials suggest that effects on breast cancer incidence persist for years after cessation of therapy.
Why this matters
- Results from large observational studies have suggested that both forms of hormone therapy are tied to increased breast cancer risk and breast cancer mortality.
- Randomized trials involved 40 US centers, beginning 1993-1998, with follow-up until 2016.
- Enrolled women (N=27,347) were aged 50-79 years without a breast cancer history.
- Compared with placebo, women undergoing hysterectomy received conjugated equine estrogens alone and those with an intact uterus received conjugated equine estrogens with medroxyprogesterone acetate.
- Primary outcome: time-specific invasive breast cancer incidence rates.
- Funding: National Institutes of Health, others.
- With 16.1 years of cumulative follow-up, in women who have undergone hysterectomy, estrogen was associated with:
- 23% reduced likelihood of a breast cancer diagnosis (HR, 0.77; 95% CI, 0.65-0.92);
- 44% decreased likelihood of breast cancer-related death (HR, 0.56; 95% CI, 0.34-0.92).
- In 18.3 years of cumulative follow-up, women with intact uterus receiving combination hormones had:
- 29% increased likelihood of a breast cancer diagnosis: HR, 1.29 (95% CI, 1.14-1.47) and
- no significant effect on risk for breast cancer-related mortality.
- Presented at a conference without peer review.