Link between breast cancer and hormone therapy confirmed

January 30, 2016

This new analysis provides even stronger evidence that recent declines in menopausal hormone therapy use are linked with lower incidence of breast cancer.

"The principal finding here is the confirmation that breast cancer rates have been moving in tandem with hormone use since 1990," said Andrew Glass, MD, the lead author on the Kaiser Permanente study and a senior investigator at Kaiser Permanente's Center for Health Research in Portland, Ore. "The advantage of this news for women is that while you can't do anything about your genetic risk or family history, you can control what goes in your body. This gives women the chance to do something that decreases breast cancer risk."

The study, conducted at Kaiser Permanente's Center for Health Research reviewed the histories of 7,386 women diagnosed with invasive breast cancer and treated at Kaiser Permanente Northwest from 1980 through 2006. Researchers used Kaiser Permanente's unique, integrated care delivery and data systems -- its tumor registry and clinical, pathology, and pharmacy data systems -- to trace connections among breast cancer rates, mammography screening, hormone therapy use, and estrogen receptor status.

"This study was made possible by the existence of Kaiser Permanente's multiple, computerized, interlinked databases. Because Kaiser Permanente is an integrated care delivery system with accurate records dating back many years, we were able to look at 26 years of data pertaining to multiple aspects of each woman's cancer," Glass said.

Other key findings of the new study include: -- Breast cancer rates increased 26 percent from the early 1980s to the early 1990s, increased an additional 15 percent through 2001, then decreased by 18 percent from 2003 through 2006. -- The 26 percent rise in breast cancer rates from 1980 to 1991 paralleled sharp increases in rates of mammography screening and use of hormone therapy, particularly of estrogen plus progestin. -- The 15 percent rise in breast cancer rates from 1992 to 2002 paralleled a continued increase in hormone therapy use, but mammography screening remained stable at 1991 levels. -- The sharp decrease in breast cancer rates starting in 2003 paralleled a 75 percent drop in hormone therapy use, but rates of mammography screening continued to remain virtually unchanged. -- These patterns of breast cancer rates were largely restricted to women over age 45 and to estrogen receptor-positive (ER+) breast cancers, the kind of breast cancer that is sensitive to hormones.

"It's been known since the late 1970s that if you had the kind of breast cancer that was sensitive to estrogen, you could shrink it by changing the woman's hormonal milieu. For example, if you gave breast cancer patients anti-hormones like tamoxifen, you could shrink the woman's breast cancer. But only recently did we begin to suspect that if you gave hormones to women free of cancer, you might start a process where they developed breast cancer," Glass said.

The study found that increases in the rate of breast cancer occurred almost entirely among ER+ cancers (breast cancers sensitive to hormones, such as estrogen receptor-positive), which made up more than 80 percent of the breast cancers diagnosed in women over the age of 45. Recent laboratory and epidemiological evidence suggests that such changes may also particularly increase the development of receptor-positive breast cancer. This study confirms that the increased use of hormones led to more receptor-positive breast cancers and that declines in hormone use led to parallel declines in receptor-positive cancers. On the other hand, the pattern for ER- cancers was quite different and is enigmatic. The rate of ER- cancers decreased from 1980 to 1995, increased from 1995 to 1999, then decreased sharply from 2000 to 2006. This pattern does not coincide with the time periods of increased mammography screening and hormone therapy use and the drop in hormone therapy use that began around 2000. The drop in hormone therapy began around 2000, confirming the suspicion that ER- cancers are not sensitive to the rise and fall in hormone therapy.

This work was funded by the National Cancer Institute, part of the National Institutes of Health. James V. Lacey Jr., Ph.D.; J. Daniel Carreon, M.S.; and Robert N. Hoover, M.D., Sc.D., of NCI's Division of Cancer Epidemiology and Genetics actively collaborated in all aspects of the study.

Kaiser Permanente's Center for Health Research, founded in 1964, is a non-profit research institution dedicated to advancing knowledge to improve health. Kaiser Permanente is America's leading integrated health plan. Founded in 1945, it is a not-for-profit, group practice program headquartered in Oakland, Calif. Kaiser Permanente serves more than 8.7 million members in nine states and the District of Columbia. Today it encompasses the not-for-profit Kaiser Foundation Health Plan, Inc., Kaiser Foundation Hospitals and their subsidiaries, and the for-profit Permanente Medical Groups. Nationwide, Kaiser Permanente includes approximately 156,000 technical, administrative and clerical employees and caregivers, and 13,000 physicians representing all specialties. For more Kaiser Permanente news, visit the KP News Center at: