(Ivanhoe Newswire) -- New evidence has surfaced that further establishes the link between combined hormone therapy and a higher likelihood of breast cancer.

In a new study published in the New England Journal of Medicine, researchers found the number of women with breast cancer dropped significantly after they stopped using estrogen plus progestin. Furthermore, this drop was not associated with a decline in the number of women getting mammograms -- a theory some have put forward to explain the lower rate of cancer.

The Women's Health Initiative (WHI) trial of estrogen plus progestin was terminated in 2002 because of the discovery of a higher incidence of breast cancer in the hormone therapy group. After a report on the study was made public, women across the United States stopped using the therapy. Following this, the incidence of breast cancer declined nationwide.

To help explain why, researchers in the NEJM study analyzed the results of the WHI trial to look for trends in breast cancer diagnoses. They found the number of cases of breast cancer was about two times higher in the group taking the therapy than in the control group, but after participants discontinued the therapy, that risk quickly declined. The results also suggest that termination of combined hormone therapy may put preclinical breast cancers into regression.

Although some experts have suggested the widespread drop in cancer incidence after the study may be explained by a decrease in mammograms, the new study results damage that theory. While the number of women who received mammograms every year remained the same after they left the study, the number of breast cancer cases in the group that had received the hormone therapy declined significantly.

SOURCE: New England Journal of Medicine, 2009;360:573-587