Major study calls into question the value of screening mammograms

Mammography does not appear to affect cancer death rates, BMJ reports

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An expansive 25-year study in the journal BMJ has found that breast cancer death rates were the same among women who underwent annual mammograms and those who did not, shedding doubt on claims that the screenings help save women's lives.

Study: Screening mammograms do not appear to change cancer death rates

For the study, one of the largest of its kind, researchers analyzed Canadian National Breast Screening Study data on 89,835 women ages 40 to 59 from six Canadian provinces.

Starting in 1980, all participants received physical breast exams each year, and half of them also received annual mammograms for five years. The researchers then monitored the woman for 25 years. (The report did not examine the use of mammograms as a diagnostic tool, which experts generally agree is valuable.)

  • With screening mammograms: The study found that 3,250 of the 44,925 women who received annual mammograms were diagnosed with breast cancer. There were 500 patient deaths from breast cancer in that group.

  • Without screening mammograms: The study found that 3,133 of the 44,910 women in the control group were diagnosed with breast cancer. There were 505 breast cancer deaths in the group.

The study notes that the death rates from all other causes also were the same among the two groups.

Overall, the researchers determined that 22% of the cancers detected by mammograms were overdiagnosed, meaning they were so slow-growing that they would have never harmed women and were treated unnecessarily. Without the screening, these women likely would have never learned they had cancer or gone through treatment, the study said.

Further, lead study author Anthony Miller, an epidemiologist at the University of Toronto's Dalla Lana School of Public Health, said overdiagnoses would have increased to about one in every three cancers if the researchers had also included a precancerous condition called ductal carcinoma in situ.

In an editorial accompanying the study, Mette Kalager—an epidemiologist and screening researcher at the University of Oslo and Harvard School of Public Health—and other experts argued that other studies that have found a benefit to screening mammograms were conducted prior to the widespread use of breast cancer drugs, such as tamoxifen, that have significantly reduced the death rate from breast cancer.

Kalager also said that many prior studies did not randomly assign participants to the intervention or control group, which is considered the gold standard in clinical trials.

What the findings mean for medicine

About 37 million mammograms are performed in the United States each year, at a cost of about $100 per mammogram, according to the New York Times. The American Cancer Society and the American College of Obstetricians and Gynecologists recommend that women receive annual mammograms starting at age 40, while the National Cancer Institute recommends that women in their 40s have the screening every one or two years.

The findings are not expected to lead to any immediate changes in mammography guidelines, and many experts "will almost certainly dispute the idea that mammograms are on balance useless, or even harmful," according to the New York Times.

However, the results provide new evidence for a growing number of experts who question the benefits of widespread screening mammography.

The American College of Radiology (ACR) immediately issued a statement arguing that the Canadian National Breast Screening Study is "deeply flawed" and "incredibly misleading." ACR said, among other things, that the research was based on "second-hand" mammogram technology that was operated by inadequately trained technicians (Kolata, New York Times, 2/11; Morin, Los Angeles Times, 2/11; Szabo, USA Today, 2/11).

The Advisory Board's take

Ingrid Lund, Imaging Performance Partnership

This study is certainly important news because mammography is such a widely used screening tool and breast cancer is the second most common cancer in the United States.

However, recent changes in the guidelines regarding the use of mammography, including the recently revised guidelines from the U.S. Preventive Services Task Force, have created a great deal of confusion among women about when they should start getting mammograms and how often. In all likelihood, these findings will only add to that confusion.

On the other hand, it's very positive that we're continuing to learn more about the pros and cons of mammography and have more data available to help individual women and their physicians make informed choices.

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