Canadian Study Questions Mammogram Screening; Findings Unlike Those of Other Studies

Canadian researchers have concluded a clinical trial that they say casts doubt on the value of annual screening mammograms for women ages 40 – 59. The study, published February 11, 2014 in British Medical Journal, found that annual screening does not reduce breast cancer deaths, but can lead to over-diagnosis, meaning cancers were found – and treated – that if undetected, would have caused no problems during the patients’ lifetime.

The findings differ from about a dozen other clinical trials that breast cancer screening experts use to make decisions about screening guidelines. And they differ from the consensus of most experts, including the US Preventive Services Task Force, which has estimated breast cancer screening reduces the relative risk of death by about 15% in women ages 40 – 59.

In the Canadian National Breast Screening Study, researchers followed almost 90,000 women for 25 years. They were randomly assigned to the mammogram group or the control group. Those in the mammogram group had a mammogram every year for 5 years, while those in the control group were not screened. Women ages 40 – 49 in the mammogram group and women ages 50 – 59 in both groups also received annual clinical breast exams. Women ages 40 – 49 in the control group received 1 clinical breast exam, and typical care from their family doctor. After 5 years, women in the study received usual care by their regular doctors, which could include mammograms at their doctor’s discretion.

During the screening period, a total of 666 cancers were found in the mammogram group versus 524 found in the control group. Most (484), but not all, of the cancers found in the mammogram group were found by the screening. Overall during the 25-year study period, about the same number of women – 3,250 in the mammogram group and 3,133 in the control group – were diagnosed with breast cancer. About the same number in each group – 500 in the mammogram group and 505 in the control group – died of breast cancer. Because more cancers were diagnosed by mammogram, but essentially the same number of women died of breast cancer, the study authors concluded that there had been over-diagnosis. They found that 22% of breast cancer cases detected through screening were over-diagnosed. They conclude that recommendations for annual breast cancer screening through mammograms be re-evaluated.

Although this study did not find an advantage for mammogram screening, a number of other studies have. Experts in the field, including those at the American Cancer Society, say the findings of this study may be due to differences in the quality of the mammograms themselves, or problems with the study design. For instance, although mammograms are meant to find cancers that are too small to be felt, most (68%) of the cancers found in the mammogram group were big enough to be felt. The study design also may have influenced the results, as women who had signs and symptoms of breast cancer (such as a breast lump) were still allowed in the study, even though screening is only meant to be used in women who don’t have signs or symptoms of the disease. Breast cancer diagnosed based on symptoms is likely to be at a more advanced stage and have worse survival than cancers found only with mammogram screening. In the study, the mammogram group included more women with symptoms of breast cancer than the control group; therefore, the study was less likely to show an advantage of mammograms.

Otis Brawley, MD, MPH, Chief Medical Officer of the American Cancer Society, said the Canadian study is one piece of information in a large volume of evidence that adds to the body of knowledge about breast cancer screening. For now, he said this new study will not change any recommendations.

The American Cancer Society recommends women age 40 and older should have a mammogram and clinical breast exam every year and should continue to do so for as long as they are in good health. The Society’s 12-person panel of experts regularly reviews all the scientific data on mammograms. The current review, which includes the Canadian study, began last August and is expected to issue updated screening guidelines later this year.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

 Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. Published February 11, 2014 in British Medical Journal. First author Anthony B. Miller, MD, University of Toronto.

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