In the 2006 subgroup analysis of WHI participants in the estrogen–progestin arm, investigators focused on HT use before enrollment in the trial. Recall that in this part of the WHI, 16,608 women with an intact uterus were randomized to conjugated equine estrogen plus medroxyprogesterone acetate or placebo. Use of the study medication was stopped after a mean follow-up of 5.6 years (mean exposure to HT: 4.4 years).

Overall, the risk of invasive breast cancer was slightly higher with combination HT than placebo (hazard ratio [HR] 1.24; 95% confidence interval [CI] 1.01–1.54).

In the 2006 report from the 2002 WHI study of estrogen–progestin HT versus placebo, investigators compared the risk of being diagnosed with breast cancer in 12,297 women who had not used HT prior to study enrollment with the risk in 4,311 participants who had previously used HT. Of the previous users, 42% reported less than 2 years of use prior to WHI enrollment, and 36% reported more than 4 years of HT prior to WHI enrollment.

The findings: Among WHI participants who had never before used HT, the use of estrogen–progestin HT in the study was not associated with an elevated risk of being diagnosed with breast cancer (HR 1.02; 95% CI 0.77–1.36). However, among previous HT users, the additional use of HT in the WHI study was associated with a risk nearly double that of placebo users (HR 1.96, 95% CI 1.17–3.27).

The reassuring results of this WHI subgroup analysis received little media attention in the United States, probably because the report appeared in a journal that has low readership in this country. WHI and other findings allow us to reassure women who have undergone hysterectomy that use of unopposed estrogen has little, if any, impact on breast cancer risk in menopausal women. This new WHI subgroup analysis, along with a recent review of European and North American data, allows ObGyns to counsel women with an intact uterus that up to 5 years of combination estrogen–progestin hormone therapy also has little, if any, impact on breast cancer risk.