Longer-term estrogen therapy is associated with increased risk of breast cancer

Chen WY et al. (2006) Unopposed estrogen therapy and the risk of invasive breast cancer. Arch Intern Med 166: 1027–1032

Researchers in the US have investigated whether long-term unopposed estrogen therapy is associated with an increased risk of invasive breast cancer. The Nurses' Health Study included women who became menopausal and underwent a hysterectomy before or during the period 1980–2002 (28,835 women as of June 2002).

During the study period 934 women were diagnosed with invasive breast cancer, 226 of whom had never received estrogen therapy and 708 of whom were current users of estrogen therapy (351 breast cancer cases in patients who had used estrogen therapy in the past were not discussed in this paper). In current estrogen therapy users, a linear increase in breast cancer risk was seen with increasing duration of use (P_trend <0.001), but the risk did not become statistically significant until estrogen therapy had been used for ≥20 years (relative risk 1.42, 95% CI 1.13–1.77). The association of breast cancer with longer-term estrogen therapy was stronger when only tumors positive for both estrogen and progesterone receptors were considered, and the association was mainly seen in women with BMI of less than 25 kg/m².

The authors conclude that estrogen therapy was associated with increased risk of breast cancer only after longer-term use (≥20 years). They suggest that alternative treatment options should, therefore, be explored for women who require longer-term estrogen therapy, such as those with osteoporosis.