Recent declines in breast cancer incidence: mounting evidence that reduced use of menopausal hormones is largely responsible.

Substantial reductions in breast cancer incidence in women 50 years old or older have been observed recently in many developed countries, and falling use of menopausal hormone therapy (HT) remains the most plausible explanation. In keeping with recent observations from the Women's Health Initiative, a report from the California Teachers Study cohort in this issue of Breast Cancer Research adds to this growing evidence. The investigators found a 26% reduction in invasive breast cancer in the cohort from 2000-2002 to 2003-2005, which accompanied an estimated 64% drop in HT use between 2000-2001 and 2005-2006. By collating individual data on the use of HT and breast cancer incidence, they also demonstrated that the decline in incidence was concentrated in women who had ceased HT use. The decline reflected a decrease predominantly in oestrogen receptor-positive tumours in the context of stable screening patterns over the study period. Millions of women continue to use HT, and these findings support carefully targeted short duration use as an important ongoing strategy to minimise breast cancer risk.