Understanding the effect of cessation of HRT may be complex. Effects may vary depending on the type of HRT used and other factors specific to how the hormones affect the body. From the data in this report, it seems that the decline in breast cancer incidence that is related to a nationwide decline in use of HRT may have has run its course, and breast cancer incidence rates may stabilize or even begin to rise again. Researchers do not yet know if this reduction in HRT use will have a long-term effect on rates, or whether reduction in hormone levels simply slowed the growth of clinically detectable tumors, in which case as HRT use stabilizes, breast cancer incidence will begin to rise again.

Several other possibilities were considered to explain the sudden decrease in new breast cancer cases, including changes in reproductive factors, rates of mammography screening, environmental exposures, and changes in diet. HRT was the only risk factor that changed substantially from 2002 to 2003 and provides a possible explanation for this trend. "Recent reports have suggested a small decline in mammography use after 2000," said Kathy Cronin, Ph.D., of the Surveillance Research Program at NCI. "Screening may play a role as well, and the contribution of mammography to the observed decline in incidence is currently being investigated."


Because this analysis is based on population statistics, the study does not prove a link between HRT and breast cancer incidence. Only a randomized clinical trial could prove causation. When the link between breast cancer and HRT was first confirmed in the WHI, which was a randomized clinical trial, women in the study were asked to discontinue their study medications (either placebo or hormones), and were encouraged to continue undergoing annual mammography. These women are still being followed, and the WHI researchers are expected to release a follow-up report later this year about the group who received HRT (estrogen and progestin) later this year. This report will provide a much higher level of evidence about the influence of HRT (and cessation of HRT) on the incidence of breast cancer.

"The decision about use of HRT is complex," says study researcher Christine Berg, M.D., from the National Cancer Institute. "While HRT provides relief from the symptoms of menopause, it may also increase one's risk of breast cancer. It is important that women meet with their doctor to discuss what decision is right for them, particularly if they are at high risk for breast cancer."

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