It is wrong to imply that estrogens alone do not cause breast cancer .Beral and colleagues wrote that the WHI studies found “remarkably similar” increased risks of HT-associated breast cancer as the Million Women Study (MWS) and the worldwide Collaborative Group’s study (in which 80% of HT users had used estrogen-only preparations). The estimated excess incidence of breast cancer with estrogens increased from 2 to 6 per 1000 users in the Collaborative study and from 1.5 to 5 per 1000 users in the MWS for 5 and 10 years of use. The excess incidence for progestin/estrogen HT increased from 6 to 18 per 1000 users in for 5 and 7 years’ use in the WHI trial and from 6 to 19 per 1000 for 5 to 10 years of use in the MWS. Progestin containing HT caused four times more breast cancer than estrogen HT in both the WHI trial and the MWS. Longer use of estrogen HT or combination HT for up to 10 years trebled breast cancer risks compared with use for up to five years.
Beral V. Banks E, Reeves G, Bull D, on behalf of the Million Women Study collaborators. Breast cancer and hormone replacement therapy: the Million Women Study. Lancet 2003;362:1330-1331.