Researchers have said that discontinuing hormone therapy may lead to decline in new breast cancer rates.
The study supports the hypothesis that stopping hormones can lead to tumour regression.
As part of the national Breast Cancer Surveillance Consortium, researchers studied 741,681 woman-years of data (with a median of 3.3 years per woman) on 163,490 women aged 50-79 who were Group Health Cooperative members and had no prior history of breast cancer.
Previous research has shown a rapid decline in new breast cancers-and also in use of hormone therapy and of screening mammography-since 2002, when the Women's Health Initiative published that breast cancer rates were higher in women taking estrogen and progestin than in those taking either a placebo or only estrogen.
Some have suggested that the decline in use of hormone therapy may have caused the fall in the breast cancer rate, perhaps by making tumors regress, Dr. Buist explained.
But others have countered that the explanation for the declines in both breast cancer and hormone use might instead be that because former hormone users are less concerned about breast cancer or see their doctors less often, they may get less screening mammography than do women who have never taken hormones.
"We set out to test this idea," Dr. Buist said, "and our results seem to refute it."
Before 2002, former users of hormone therapy had lower rates of screening mammography than did current users.
"But we found that this is no longer true," she said. Indeed, former users had the same or even slightly higher screening rates than current users.
"We concluded that differences in rates of screening mammography don't explain the declines in rates of the incidence of invasive breast cancer among women who've stopped using hormone therapy," Dr. Buist added.
The study has been e-published in Cancer Epidemiology, Biomarkers, 'n' Prevention.