As soon as women quit hormone therapy, their rates of new breast cancer decline, supporting the hypothesis that stopping hormones can lead to tumor regression, according to a report e-published in Cancer Epidemiology, Biomarkers, & Prevention.

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.

“This is the first study to look over time at screening mammography use among individual women by their hormone therapy status linked with their breast cancer diagnoses,” said lead author Diana S.M. Buist, PhD, a senior investigator at Group Health Research Institute.

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 said.
The National Cancer Institute, which supports the Breast Cancer Surveillance Consortium, funded this study. The collection of cancer data was also supported in part by several state public health departments and cancer registries throughout the United States.

In addition to Dr. Buist, the other co-authors were Diana L. Miglioretti, PhD, Rod Walker, MS, and Erin J. Aiello Bowles, MPH, of Group Health Research Institute; Walter Clinton, PhD, of Veterans Affairs (VA) Puget Sound Healthcare System; Patricia A. Carney, PhD, of Oregon Health & Science University, in Portland; Stephen H. Taplin, MD, MPH, of the National Cancer Institute; Tracy Onega, PhD, of Dartmouth Medical School, in Lebanon, NH; and Karla Kerlikowske, MD, of the University of California, San Francisco.

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Group Health Research Institute
Group Health Research Institute is the research arm of Seattle-based Group Health Cooperative, a consumer-governed, nonprofit health care system. Founded in 1947, Group Health Cooperative coordinates health care and coverage. Group Health Research Institute changed its name from Group Health Center for Health Studies in 2009. Since 1983, the Institute has conducted nonproprietary public-interest research on preventing, diagnosing, and treating major health problems. Government and private research grants provide its main funding.

Breast Cancer Surveillance Consortium
The Breast Cancer Surveillance Consortium (BCSC) is the nation’s largest and most comprehensive collection of breast cancer screening information. It’s a research resource for studies designed to assess the delivery and quality of breast cancer screening and related patient outcomes in the United States. The BCSC is a National Cancer Institute-funded collaborative network of seven mammography registries with linkages to tumor and/or pathology registries. The network is supported by a central Statistical Coordinating Center. Currently, the Consortium’s database contains information on over 9.5 million mammographic examinations, 2.3 million women, and 114,000 breast cancer cases (95,000 invasive cancers and 19,000 ductal carcinoma in situ).

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Rebecca Hughes
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Group Health Research Institute