Women who start hormone replacement therapy (HRT) at the menopause have a higher breast cancer risk than women who wait for five years or more, research shows. Authors of the study, published online in the Journal of the National Cancer Institute, said that women who left at least a five-year gap had little or no increased risk of breast cancer no matter which HRT they used and for how long, and whether or not they were overweight.
The researchers, Valerie Beral and her colleagues from Oxford University, analysed data from the Million Women Study. They estimated the relative risk of breast cancer in 1.13m women who had and had not used hormone therapy; who used either opposed or unopposed oestrogen; and who started HRT at different intervals after onset of menopause.
They found an association between starting HRT at the time of the menopause and an increased risk of breast cancer, compared with starting it later. “A new finding of this study, which has been little investigated previously, is that the interval between menopause and starting hormonal therapy has a substantial effect on breast cancer risk,” they said.
They reported: “Among current users of oestrogen-only formulations, there was little or no increase in risk if use began five years or more after menopause, but risk was statistically significantly increased if use began before or less than 5 years after menopause. A similar pattern was observed among current users of oestrogen–progestin formulations.”
Two earlier studies had suggested such an association but only in certain subgroups of women. The authors of this new research said their work showed that it was relevant to all women, regardless of bodyweight: “In this large study, we found greater risks of breast cancer if hormonal therapy use began either before or soon after menopause than after a longer gap; and this pattern of risk was seen across different types of hormonal therapy, among women who used hormonal therapy for either short of long durations, and also in lean and in overweight and obese women.”
Authors of an accompanying editorial said the results from the Oxford research backs up evidence from the US study, the Women’s Health Initiative (WHI). They said that the similarities between the patterns of breast cancer risk in these two large studies increase the probable validity of the results, especially as the studies’ methodologies were quite different.
But whereas WHI had found little additional risk of breast cancer with unopposed oestrogen, the Million Women Study did find a significantly increased risk, except in overweight and obese women. The editorial writers conclude that “the question of the effect of oestrogen-only formulation use on breast cancer risk in postmenopausal women, even with longer-term hormone use, still stands unanswered.”