Evidence from the Million Women Study, an analysis used to establish that hormone replacement therapy (HRT) causes breast cancer, is not definitive, according to an article published online on 16 January in the Journal of Family Planning and Reproductive Health Care.
The Million Women Study tracked new cases of breast cancer arising in women who had breast cancer screening mammograms in the U.K. between 1966 and 2001. It ultimately studied nearly 1.13 million women and more than 4.05 million women years of follow-up.
An analysis conducted by a research team from four countries identified several design flaws in the study, which was one of three major research initiatives that questioned the long term safety of HRT.
One design flaw that would have skewed the findings was that women who had cancers detected within a few months of the study's start, and would have already been present when the women enrolled in the study, were not excluded, according to lead author Dr. Samuel Shapiro, a visiting professor of epidemiology of the University of Cape Town in South Africa.
The authors also noted that crucial data were missing, citing the example of the third Million Women Study report in which follow-up data for 57% to 62% of the participants were missing.
