The debate over the value of preventive screening heated up early this year after a study published in The British Medical Journal reported that annual mammograms did not reduce deaths from breast cancer.
The Canadian National Breast Screening Study evaluated data from 25 years on 90,000 women 40 to 59 years old in one of the largest studies of its kind.
In the study, published in February, Canadian women were randomized to receive regular or no mammograms. Women 40 to 49 in the no-mammogram group were given a single breast exam followed by usual care by their family doctors. Women of that age in the mammography group and women 50 to 59 in the mammogram and no-mammogram groups got annual breast exams.
Researchers found that death rates from breast cancer and from all other causes did not differ for women in the trial based on whether they got annual mammograms.
Researchers also found that mammography had a 22 percent overdiagnosis rate, which resulted in one of every 424 women who had a mammography-detected breast cancer getting unnecessary treatment with surgery, radiation or chemotherapy.
The researchers concluded that "the data suggest that the value of mammography screening should be reassessed."
The ongoing debate about preventive screening has led to conflicting recommendations for women of average risk.
The American Cancer Society advises average-risk women to get an annual mammogram starting at 40. The U.S. Preventive Services Task Force, an independent panel of national experts convened by the federal government, recommends mammograms for women every other year starting at 50. Both groups are in the process of reviewing research and plan to update their recommendations.
Dr. Carol H. Lee, a radiologist who specializes in breast imaging at Memorial Sloan Kettering Cancer Center in New York, said most large population studies, including a Swedish study of more than 130,000 women 40 to 74 years old published in the journal Radiology in 2011, have shown that average-risk women screened with mammography were less likely to die from breast cancer.
In the U.S., age-adjusted death rates from breast cancer fell from 33.23 per 100,000 in 1989 to 21.92 in 2010, a 34 percent decline, according to the American Cancer Society.
That decrease is due to improvements in treatment, greater awareness about abnormalities in breasts and preventive mammography, in that order, said Dr. Otis Brawley, the cancer society's chief medical officer.
Lee said: "Some would say (the declining death rate is due to) treatment. But we know that treatment at early-stage disease is more successful than treating late-stage diseases."
Although he advocates annual screening, Brawley cautioned that "women shouldn't believe all the hype about mammograms."
"Women need to realize that a mammogram is not fail-safe," he said. "It misses cancers that we wished it would find, and it finds cancers we wished it hadn't. It causes some women to get treatment who don't need it, but by treating everybody, we have good science to show that it does save lives."
Dr. Peter M. Jokich said the value of mammography is that it can detect many small breast cancers before they can be felt.
"I think the benefit (of annual mammograms) greatly outweighs the risks," said Jokich, director of the division of breast imaging at Chicago's Rush University Medical Center. "If you are going to find cancer early, you have to be willing to live with false positives and benign biopsies."
Mixed messages on mammograms: Should you be screened?