Patient treatment records from a large HMO show that the recent decline in breast cancer rates is linked to a sharp drop in use of hormone replacement therapy and not to declines in mammography, as many researchers had speculated, researchers said Monday.
Dr. Andrew G. Glass and his colleagues at Kaiser Permanente Northwest in Portland, Ore., found a drop in breast cancer rates among their patients from 2003 to 2006 even though mammography rates remained virtually constant, they reported in the Journal of the National Cancer Institute.
A lowered mammography rate means fewer breast cancers are detected. Researchers are not sure why that rate declined.
By using a patient population for whom the mammography rate remained constant, “we were able to disentangle mammography from hormones,” Glass said.
In an editorial in the same journal, Dr. Donald A. Berry and Dr. Peter M. Ravdin of the University of Texas M.D. Anderson Cancer Center in Houston agreed that declines in mammography could not be responsible for the bulk of the decline in breast cancer.
“The only known factor that would seem to explain the precipitous drop in incidence is the sharp decrease in use of menopausal hormone therapy,” they wrote.
No one is suggesting that estrogen triggers the formation of breast tumors. Rather, most researchers think that it accelerates the growth of existing tumors.
Stopping hormone replacement therapy slows the growth of small tumors, delaying their detection and producing an apparent decline in incidence, Berry and Ravdin speculated.
Prescriptions for estrogen and progestin fell by nearly half in 2003 after 2002 results from the Women’s Health Initiative linked an increased breast cancer risk to hormone replacement therapy.
