Summary: New research by American Cancer Society scientists supports a recent report that links a drop in breast cancer rates between 2002 and 2003 to a simultaneous decline in the use of hormone therapy. But the new work also shows that breast cancer rates were going down even before 2002 -- at the same time that mammogram use was leveling off. That suggests mammogram use was also a factor in the breast cancer decline.

Why it's important: Breast cancer is the most common cancer among American women (other than skin cancer), so a drop in breast cancer rates is good news. If researchers can learn what factors caused breast cancer rates to go down, they may be able to develop ways to keep the trend going. By the same token, understanding what's behind the decline may help researchers clue in to other trends that could jeopardize progress. That issue is key when it comes to mammograms. If mammogram use goes down, that means fewer breast cancers are being found -- not necessarily that fewer women are getting breast cancer.

What's already known: The drop in breast cancer rates is a relatively new development. Rates actually increased steadily -- by almost 40% -- between 1980 and 1998. There are several reasons for this increase. One of the most important factors was better early detection of breast cancer. Mammograms were coming into wider use during this time, so many breast cancers were being found that otherwise might have gone undetected for months or even years. Mammograms can find breast tumors when they are too small to be felt with a physical exam.

Earlier this year, a team from the University of Texas MD Anderson Cancer Center reported that breast cancer cases dropped sharply between 2002 and 2003, and leveled off between 2003 and 2004. The most likely explanation for that, they said, was that many women abruptly stopped using hormone replacement therapy in 2002 because the large Women's Health Initiative study had just linked it to breast cancer, heart disease, and other problems.

The new ACS study suggests quitting hormone therapy wasn't the only factor behind the drop in breast cancer rates.

How this study was done: ACS epidemiologists led by Ahmedin Jemal, DVM, PhD, used a large national database to examine breast cancer rates in women 40 and older between 1975 and 2003. They looked at tumor size, stage, and whether it had receptors for estrogen (ER-positive) or progesterone (PR-positive). Tumors with hormone receptors grow in response to estrogen (such as that delivered by hormone therapy) and can be treated with common estrogen-blocking drugs like tamoxifen and aromatase inhibitors.

What was found: The ACS analysis saw the same sharp drop in breast cancer cases between 2002 and 2003, as reported by the MD Anderson researchers. This decline was greatest among women 50-69 years old (those most likely to use hormone therapy) with ER-positive and PR-positive tumors (the kind fueled by hormone therapy). That adds weight to the idea that quitting hormone therapy was behind this decline.

But quitting hormone therapy can't explain the other major finding from the ACS study. Jemal and his colleagues found that breast cancer rates (especially for small tumors that had not spread) actually started dropping in 1999 for all women 45 and older. That's well before the link between hormone therapy and health problems had been established.

The bottom line: The most likely explanation for the earlier decline, the ACS team says, is that mammography use leveled off around that time, after about 12 years of steady increases. In each of the years between 1999 and 2003, about 70% of women reported getting a regular mammogram.

But that means that part of the decline in breast cancer cases may be temporary -- the result of delayed detection, rather than an actual decrease in incidence.

The American Cancer Society recommends that women get a mammogram every year, starting at age 40. Doing this offers the best chance for finding a breast tumor when it is still small and treatments are more likely to be successful.

Citation: "Recent Trends in Breast Cancer Incidence Rates by Age and Tumor Characteristics among US Women." Published online May 3, 2007 in Breast Cancer Research. First author: Ahmedin Jemal, DVM, PhD, American Cancer Society.