Guilt by Association
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By Damaris Christensen

Guilt by Association

A drop in breast cancer cases may be linked to less use of hormone replacement therapy

By Damaris Christensen


The number of women who develop breast cancer each year has dropped dramatically—and a multi-institutional team of researchers believes a major reason is that millions of women have stopped using hormone replacement therapy (HRT).

In July 2002, a large study known as the Women’s Health Initiative (WHI) linked the use of HRT at menopause to an increased risk of breast cancer, heart disease, stroke and other health problems. Within months, millions of women stopped taking estrogen and progestin pills.

Recently, researchers have reported that rates of breast cancer diagnoses in 2003 were 7 percent lower than in 2002—the largest drop in breast cancer incidence observed in a single year. The most dramatic decline in breast cancer incidence took place between July 2002 and December 2003, when the incidence of breast cancer among women older than 50 dropped 14 percent. (The registry data includes women of all races, though the researchers have not yet reported on any differences among these groups.)

“The trend is really quite clear,” says Donald Berry, a biostatistician and breast cancer researcher at the University of Texas M. D. Anderson Cancer Center in Houston, and a co-author of the new study.  

The researchers, who reported their findings in December 2006 at the 29th Annual San Antonio Breast Cancer Symposium in Texas, examined the records of women with newly diagnosed breast cancer in registries from nine locations nationwide. The decline in incidence was most evident among patients over 50—postmenopausal women who were most likely to be taking HRT—and in cancers that were estrogen-receptor positive, meaning that estrogen could fuel their growth. Those results, together with evidence that about half the women on HRT quit using the hormones shortly after the 2002 WHI announcement, add up to “a smoking gun,” says Berry.

Christina Clarke, an epidemiologist with the Northern California Cancer Center in Fremont, calls the decline in breast cancer incidence “absolutely unprecedented.” Confirming the pattern, additional cancer registries are recording similar declines, she adds. Clarke herself has reported that breast cancer incidence dropped in California in 2003 and in 2004, following dramatic declines in hormone use.

The new study results are good news, says oncologist Eric Winer, the director of the Breast Oncology Center at the Dana-Farber Cancer Institute in Boston. While the researchers did not prove that fewer women developed breast cancer as a result of fewer taking hormones, the association is “very plausible,” he says. “We know that being on hormone replacement therapy causes breast cancer; we know that anti-estrogen therapy can reduce tumor size in hormone-sensitive cancers; and at one point in time, the incidence of breast cancer dropped pretty dramatically.”

Winer cautions that the observed reduction in cancer incidence may not lead to an equivalent decline in death rates from breast cancer, since hormone-sensitive tumors tend to be the ones most responsive to treatment.

Carolina Hinestrosa, the executive vice president of the National Breast Cancer Coalition, a nonprofit organization based in Washington, D.C., says the apparent decline in breast cancer “is news we’ve wanted to see for many years.” Still, she cautions that further studies are needed to confirm the drop and understand its causes. “It is surprising to see a decline so soon,” she adds.

According to Berry, the fast decline has a likely explanation: HRT prompted or sped the development of very small cancers; without the hormones, similar cancers now remain too small to be detected. Breast cancer incidence may rise again in a few years if halting the hormones simply slowed down cancer growth, he notes. However, “If it is stopping the growth, we’re going to stay at this lower incidence or even continue to [see a] decline.”