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A rapid, dramatic drop in breast cancer rates between 2002 and 2003 leveled off in 2004, researchers say, lending more weight to the theory that a decline in the use of hormone replacement therapy (HRT) may be behind the trend. Stopping HRT may have stopped or slowed the growth of undetected breast tumors, the theory goes.

Researchers from the University of Texas MD Anderson Cancer Center first reported the breast cancer decline last December, at the San Antonio Breast Cancer Symposium. Their calculations using national cancer data showed breast cancer incidence dropped by nearly 7% between 2002 and 2003. That's the same period when hormone use fell off sharply after a large clinical trial found that HRT with estrogen and progestin could raise the risk of breast cancer.

Now, they've published those results, along with some additional data. Their report appears in this week's New England Journal of Medicine.

The new findings show that breast cancer rates stabilized at that new lower level between 2003 and 2004 -- as did use of HRT. Moreover, the decline in breast cancer incidence was seen only in women 50 or older (those most likely to have used hormone therapy) and was greater for estrogen-receptor-positive tumors (the kind fueled by hormone therapy) compared to those with no estrogen receptors.

This suggests that declining hormone use was the primary factor behind the drop in breast cancer, according to lead researcher Peter Ravdin, PhD, MD, and his colleagues.

That doesn't mean it was the only factor, though. A decline in the mammography rate could also have played a role, Ravdin's report acknowledges. If fewer women get mammograms, fewer breast cancers are found.

The American Cancer Society recommends that women get annual mammograms beginning at age 40 to screen for breast cancer. These breast x-rays can often find tumors before they are big enough to be felt in a physical exam, and finding breast cancer early offers women the best chance of successful treatment.

Yet despite the benefits, mammography rates dropped by a little over 3% between 2000 and 2003, according to Ravdin's report. That decline is probably not large enough to account for such a dramatic drop in breast cancer rates, the report says, but it could have been a contributing factor.

American Cancer Society experts agree mammograms may be a factor. Their research, which will be published in an upcoming issue of the journal Breast Cancer Research, also shows a decline in the use of mammography.

"This results in delayed detection of small breast cancers rather than a true decrease in incidence," says ACS epidemiologist Ahmedin Jemal, PhD.

Indeed, Ravdin and other researchers can't say for sure that the decline in breast cancer incidence is permanent.

If stopping HRT only slowed the growth of existing, but hidden, cancers, the breast cancer rate could creep up again as those cancers resume growing and become detectable. Likewise, cancers that were not found because a woman skipped a mammogram will eventually be detected by other means and be counted.

Citation: "The Decrease in Breast-Cancer Incidence in 2003 in the United States." Published in the April 19, 2007, New England Journal of Medicine (Vol. 356, No. 16:1670-1674). First author: Peter M. Ravdin, PhD, MD, University of Texas MD Anderson Cancer Center.