One of the most extensive studies on mammograms to date finds that the procedure isn’t any more effective in preventing breast cancer deaths than a traditional physical breast exam. In fact, according to the researchers from the University of Toronto, mammograms actually lead to significant over-diagnosis of the disease — which can result in unnecessary, costly treatments for cancers that may have receded without an intrusive intervention.
Mammograms are used to detect cancerous breast tumors that are too small to be seen by the naked eye or felt in a standard breast exam. The randomized control study was massive in scope, employing nearly 90,000 women aged 40 to 59 who were tracked over the course of 25 years. Approximately 45,000 of these women were told to get mammograms every year — as is recommended by the American Cancer Society for all women 40 and older — for five years. The other half were told not to get any mammograms at all.
The results were striking. Over the 25-year period, 3,250 women were diagnosed with breast cancer among the group that received the mammograms, while 3,133 women were diagnosed in the non-mammogram group. But 500 of the women diagnosed after a mammogram died of breast cancer, while 505 women who just had traditional exams died from the disease — a statistically insignificant difference of one percent in mortality rate.
“[O]ur data show that annual mammography does not result in a reduction in breast cancer specific mortality for women aged 40-59 beyond that of physical examination alone or usual care in the community,” concluded the study authors. “The data suggest that the value of mammography screening should be reassessed.”
Although mammograms did almost nothing to lower mortality, the procedure did cause an abundance of over-diagnoses. In fact, 22 percent of the breast cancers detected by mammograms in the trial were over-diagnosed, “representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.” Since nearly 37 million mammograms are conducted in the U.S. every year, the new study suggests that many women are being diagnosed and treated for breast cancers that have a negligible tangible effect on their actual health.
The authors did note that there was an appreciable difference in the survival rate among those who had mammograms and those who did not (79.6 percent versus 62.8 percent, respectively). However, those differences are largely attributable to several statistical kinks, noted the authors.
The mortality rate measures the number of people who die from a specific disease every year. But the survival rate is defined as the percent of people who survive a disease they they’ve been diagnosed with for a specified amount of time. Unlike the mortality rate, survival rates can be artificially bolstered by making an earlier diagnosis of a disease through a screening, even if that diagnosis never leads to death or another adverse health outcome — a phenomenon known as “lead time bias.” Pennsylvania State University has a helpful graphic illustrating what this bias looks like:
CREDIT: Pennsylvania State University
The new study is likely to add to the controversy surrounding mammograms’ efficacy, which has been hotly debated among public health experts in recent years. A separate study by the University of California, San Francisco released just last week concluded that biannual mammogram screenings are just as effective for older women as yearly ones. A 2012 report published in the New England Journal of Medicine found that “screening mammography has only marginally reduced the rate at which women present with advanced cancer… and that screening is having, at best, only a small effect on the rate of death from breast cancer.”
One major reason cited by the University of Toronto researchers for the negligible differences in mortality between women who get mammograms and those who don’t is the increasing effectiveness of breast cancer prevention drugs such as tamoxifen and raloxifene. Last month, the Obama administration announced that these medications must be covered by insurance plans at no additional cost to women who are at risk for breast cancer.
According to the New York Times, the American Cancer Society is currently in the process of assessing the scientific literature on the effectiveness of mammograms. The group is slated to issue revised recommendations later this year.