A study that casts doubt on the usefulness of mammograms for long-term cancer survival may have grabbed headlines recently. But health experts still maintain that annual mammograms for women over age 40 will save lives.
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The results, published last week in the British Medical Journal, were a 25-year follow-up of the Canadian National Breast Screening Study. They concluded that annual mammograms for women between the ages of 40 and 59 do not reduce deaths from breast cancer detected through physical exams, if post-surgical treatments such as chemotherapy, radiation or hormone treatments are available.
The screening data were originally collected in Canada between 1980 and 1985. Researchers assigned close to 90,000 women ages 40 to 59 to either an experimental group or a control group. The experimental group received annual mammograms; the control group received none.
Flaws in the study
Radiologist Alice Rim, MD, Section Head of Breast Imaging in the Department of Diagnostic Radiology at Cleveland Clinic, says the study has two major flaws, prompting deep doubts among the medical community about the findings.
One flaw is that the participating women were not randomly assigned to the experimental and control groups, Dr. Rim says. Initially, the women were given a clinical exam by nurses, so that they knew which women had breast lumps — many of which were cancers — and which women had large lymph nodes in their armpits — many of which indicated advanced cancer. Investigators already knew who had large incurable cancers before they assigned the women to either group.
“Most of the women with cancer went into group that received mammograms,” Dr. Rim says. “Of course, they did not do well.”
If researchers assigned participants by chance to separate groups, then the groups would be similar and the treatments they receive could be compared objectively. This kind of assigning, part of a randomized controlled trial, is the gold standard for a clinical trial. The Canadian study did not follow this protocol, Dr. Rim says.
The other problem with the study is that the mammogram equipment was of poor quality and the technicians were not properly trained in how to use them.
“With the corrupted randomizing and poor equipment, this trial has always been disregarded by the scientific community,” Dr. Rim says.
What you need to know
The American College of Radiology and the Society of Breast Imaging in a joint statement called the newly published study “an incredibly misleading analysis based on the deeply flawed and widely discredited Canadian National Breast Screening Study.”
The professional organizations said the results should not shape breast cancer screening policy, “as this would place a great many women at increased risk of dying unnecessarily from breast cancer.”
Dr. Rim says the recommendations by the American Cancer Society are still the most authoritative and supported by science: Yearly mammograms starting at age 40 and continuing for as long as a woman is in good health. A woman with risk factors, such as family history, should consult her doctor about whether she should have mammograms at a younger age.
“The new study is not a reason to change the game plan,” Dr. Rim says.
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