Vrouwen die hormonen, oestrogeen en progestin nemen om de effecten van de overgang weg te nemen krijgen daarbij een significant groter risico op het krijgen van borstkanker en als ze borstkanker krijgen hebben deze vrouwen in vergelijking met vrouwen die een placebo namen een vorm van borstkanker waarbij vaker de lymfklieren ook al zijn aangetast ((81 [23.7%] vs 43 [16.2%]). Het verschil was 92 vrouwen meer per jaar uit een groep van de hormonen kregen borstkanker dan uit de groep die een placebo kregen. (385 vrouwen [0.42% per jaar] vs 293 vrouwen [0.34% per jaar]). Het is al veel langer bekend dat nadat bekend werd dat hormoontherapie (HRT) het risico op borstkanker vergroot en vooral in Amerika veel vrouwen daarmee stopten de incidentie van borstkanker sterk afnam. Nu heeft dus een placebo gecontroleerde studie bewezen dat hormonen in de overgang het risico op borstkanker vergroot.

In the WHI, 16,608 postmenopausal American women, 50 to 79 years of age, who had not undergone hysterectomy were randomly assigned to receive either combined conjugated equine estrogens 0.625 mg/day plus medroxyprogesterone acetate 2.5 mg/day, or placebo.

After the original trial completion date, reconsent was required for continued follow-up for breast cancer incidence, and was obtained from 12,788 (83%) of the surviving participants, report the authors, led by Rowan T. Chlebowski, MD, PhD, from the Los Angeles Biomedical Research Institute at Harbor–UCLA Medical Center in Torrance, California.

The WHI authors performed intention-to-treat analyses and found that combined estrogen-plus-progestin hormone therapy increased the incidence of invasive breast cancer, compared with placebo (385 cases [0.42% per year] vs 293 cases [0.34% per year]).

They also found that significantly more women in the combined hormone therapy group had breast cancers with positive lymph nodes than in the placebo group (81 [23.7%] vs 43 [16.2%]).

The authors point out that this finding conflicts with observational studies. With some exceptions, observational studies "have associated combined hormone therapy use with an increase in breast cancers that have favorable characteristics" and are at a lower stage than breast cancers in women not receiving the therapy, write the study authors.

Observational studies have also found that women with breast cancer receiving combined hormone therapy survived longer than those not receiving therapy. This was not the case with the WHI's latest data, report the authors.

In the WHI data, there were more deaths directly attributed to breast cancer among the hormone group than among the placebo group (25 deaths [0.03% per year] vs 12 deaths [0.01% per year]; hazard ratio, 1.96; 95% confidence interval, 1.00 - 4.04; P = .049).

These deaths represent 2.6 and 1.3 deaths per 10,000 women per year, respectively, the authors write.