February 12, 2014 — Breast cancer death rates were the same among women who underwent regular mammograms and those who did not, shedding doubt on claims that the screenings help save women's lives, according to an expansive 25-year study in the journal BMJ, the New York Times reports (Kolata, New York Times, 2/11).
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 (Morin, Los Angeles Times, 2/11).
About 37 million mammograms are performed in the U.S. each year, at a cost of about $100 per mammogram, according to the New York Times. Nearly 75% of women ages 40 and older say that they have had a mammogram within the past year (New York Times, 2/11).
The researchers analyzed data from the Canadian National Breast Screening Study that included medical records of 89,835 women ages 40 and 59 from six Canadian provinces. All participants received annual physical breast exams, while half of them also received annual mammograms for five years, beginning in 1980.
The report did not examine the use of mammograms as a diagnostic tool, which experts generally agree is valuable.
The researchers followed the women for 25 years. They found that 3,250 of the 44,925 women who received annual mammograms were diagnosed with breast cancer, as were 3,133 of the 44,910 women in the control group. There were 500 patient deaths from breast cancer in the mammography group, compared with 505 breast cancer deaths in the control group, according to the study (Los Angeles Times, 2/11). Death rates from all other causes also were the same among the two groups (New York Times, 2/11).
Overall, the study found 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 (Szabo, USA Today, 2/11).
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 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.
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 (New York Times, 2/11).
The American College of Radiology 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 (Los Angeles Times, 2/11).