Hormone therapy has differing effect on breast cancer incidence

Rowan Chlebowski, MD, PhD, City of Hope National Medical Center, Duarte, CA, presented updated results from two randomized trials conducted within the Women’s Health Initiative.

These were two separate randomized, placebo-controlled, prospective trials that included women 50-79 years old. In the first, women with no prior breast cancer and clear mammograms received conjugated equine estrogens (CEE) plus medroxyprogesterone acetate (MPA). In the second, women were treated with CEE only.

Interestingly, these two treatments had opposite effects on the incidence of breast cancer. CEE plus MPA treatment significantly increased the incidence of breast cancer. These results held over the long term, persisting for over 10 years after treatment discontinuation. Furthermore, this increased incidence was even greater in women previously treated with hormone therapy. CEE used alone, however, significantly decreased the incidence of breast cancer (again over the long-term, persisting 10 years after discontinuation).

These results were shared at SABCS 2019.