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Incidence of breast cancer in 2003 in US women aged 50 or older dropped by 6.7%, a year after many women stopped using hormone replacement, says a report in the New England Journal of Medicine (2007;356:1670-4, http://content.nejm.org/cgi/content/abstract/356/16/1670).
Women's use of hormone replacement fell substantially after the Women's Health Initiative study showed an increased risk of cardiovascular events and breast cancer in women using a combination of oestrogen and progestogen called Prempro (JAMA 2002;288:321-33, 366-8).
Incidence of breast cancer fell sharply between 2002, when the JAMA study results were released, and levelled off in 2003, said researchers from MD Anderson Cancer Center, the US National Cancer Institute, and the Los Angeles Biomedical Research Institute. The decrease occurred only in women aged at least 50 and was more evident in cancers that were oestrogen receptor positive. Their study, based on data from nine cancer registries in the surveillance epidemiology and end results (SEER) programme of the National Cancer Institute, includes about 9% of the US population.
In contrast, in the 1990s, incidence of breast cancer increased in older women by about 0.5% a year. Although other factors might have had an effect, “only the use of hormone-replacement therapy changed substantially between 2002 and 2003,” the authors write.
In 2001-4, when incidence in older women was falling, breast cancer among women younger than 50 rose by 1.3%.
By 2005, prescriptions for Prempro, the combined hormone replacement therapy pill, had dropped 90%, and those for conjugated equine oestrogens by 60%, Donald Berry, chairman of biostatistics at MD Anderson Cancer Center in Houston, Texas, told the BMJ.
“It's pretty likely that hormone replacement therapy doesn't cause breast cancer but fuels it,” he said. There is “a possibility” that some breast cancers might remain indolent but are encouraged to grow by hormone replacement, he said. When a woman stops hormone replacement, the cancer might slow its growth, or stop, or even regress. “The rapidity of change [of incidence in the study] suggested that clinically occult breast cancers stopped progressing or even regressed after discontinuation of the therapy,” the authors wrote.
As for the effect of hormones in oral contraceptives, which the study did not look at, he told the BMJ, “We don't know.” The effect of lower doses of hormones and the effect in younger women are unknown.
Stopping postmenopausal hormone replacement might delay the occurrence of clinically detectable tumours or might lead to long term reduction. If so, after the use of hormone replacement therapy stabilises, the incidence of breast cancer may rise again. An answer may come from a follow-up study to be reported later this year of women in the Women's Health Initiative study who continued to receive annual mammography after stopping their treatment.