Falling breast cancer rates: What's behind the numbers?

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Falling breast cancer rates: What's behind the numbers?

Breast cancer rates fell after women stopped HRT. A Mayo Clinic expert looks at why.

Photo of Sandhya Pruthi, M.D.
Sandhya Pruthi, M.D.

Breast cancer rates among women age 50 and older were almost 9 percent lower in 2004 than they were in 2001, according to an analysis published in the New England Journal of Medicine on April 19, 2007. The sharpest decline — 7 percent — came in 2003.

Interestingly, that was also the year after a major study of combination hormone replacement therapy (HRT) in postmenopausal women was cut short when it became apparent that the treatment was actually increasing both cardiovascular and breast cancer risk. This led to a significant drop in use of HRT by postmenopausal women in the United States.

If HRT use hadn't fallen so sharply in 2002, would thousands more women have been diagnosed with breast cancer in 2003? Were there possibly other reasons that could explain this decline in number of new breast cancers? Sandhya Pruthi, M.D., a breast-health specialist at Mayo Clinic, Rochester, Minn., explains the controversy.

Some news reports said the rate of breast cancer actually fell 15 percent in 2003. Where did they get that number?

The decrease in breast cancer incidence was as high as 15 percent for women ages 55 to 59 and for women ages 65 to 69. The decrease was greatest in women diagnosed with estrogen receptor positive tumors, especially those ages 50 to 69. Using this figure is misleading because it doesn't represent the rate of decrease for women overall.

What other factors might have led to fewer breast cancers being diagnosed in 2003?

No single factor has been implicated as the cause of all breast cancers. Instead, the disease is most likely due to multiple factors. These factors include your family history and your exposures to reproductive hormones (the ones your body produces as well as the ones you take in) and environmental carcinogens. Diet and lifestyle also play a role.

Further, as HRT use declined, it is possible women were not receiving mammograms. From 2000 to 2003, after a few major reports questioned the value of recommending screening mammography to all women over age 40, mammogram rates did decline. If fewer mammograms were performed in 2003, it stands to reason that fewer cancers were detected, which could lead, in part, to a decrease in the breast cancer rate.

Women also increased the amount of calcium in their diets in recent years, another trend that might have affected the 2003 breast cancer rate. Results from another Women's Health Initiative (WHI) study showed a modest decrease in breast cancer risk in women who used calcium when compared with women who didn't use calcium.

Other medications may have had an impact on the breast cancer rate as well. A separate WHI study linked use of anti-inflammatory medications to a lower risk of breast cancer. Likewise, increased use of the osteoporosis drug raloxifene (Evista), which has also been shown to have a protective effect against breast cancer, could have played a role.

Data from 2004 weren't included in the first report of the 2003 decline in breast cancer rates. Does having this data now seal the case for an HRT connection?

No, but it reinforces the importance of monitoring trends in breast cancer incidence. More data will be needed before researchers can demonstrate a direct cause-and-effect relationship. If the decline persists over the next three to five years, the case will be bolstered. But new questions may arise.

The picture will be clearer when a follow-up report on the WHI study participants who stopped combination HRT in 2002 comes out. That report is expected later this year. Analyzing clinical outcomes in this group will provide more evidence about how stopping combination HRT has had a long-term effect on the numbers of new breast cancer cases.

Last Updated: 04/23/2007
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