The study is based on results from the Canadian National Breast Screening Study. For this research, 89,835 women ages 40 to 59 were randomly assigned to receive either annual mammograms for five years, or no mammograms during the study period. All of the women received breast exams by trained nurses. The women were followed for up to 25 years to see which of them died of breast cancer.
Based on the data, the authors report that there was no difference between those who had screening mammography and those who didn’t in terms of their likelihood of dying from breast cancer. The authors conclude:
Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available. Overall, 22% (106/484) of screen detected invasive breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.
Put simply, this means that one in five cancers do not pose a deadly threat, yet these women may still undergo treatment, including surgery, chemotherapy and radiation.
A New York Times article this week summarizes the research, and provides some commentary from doctors. Among them, Dr. Russell P. Harris, a screening expert and professor of medicine at the University of North Carolina, Chapel Hill, commented that the results “will make women uncomfortable, and they should be uncomfortable.”
“The decision to have a mammogram,” he added, “should not be a slam dunk.”
New York Times reporter Gina Kolata explains that the number of cancers treated unnecessarily would be even higher if a precancerous condition had been included in the study:
Many cancers, researchers now recognize, grow slowly, or not at all, and do not require treatment. Some cancers even shrink or disappear on their own. But once cancer is detected, it is impossible to know if it is dangerous, so doctors treat them all.
If the researchers also included a precancerous condition called ductal carcinoma in situ, the overdiagnosis rate would be closer to one in three cancers, said Dr. Anthony B. Miller of the University of Toronto, the lead author of the paper. Ductal carcinoma in situ, or D.C.I.S., is found only with mammography, is confined to the milk duct and may or may not break out into the breast. But it is usually treated with surgery, including mastectomy, or removal of the breast.
These findings are unlikely to result in any immediate change in what doctors recommend to women, and are likely to cause controversy among experts.
An accompanying editorial in BMJ, “Too Much Mammography,” explains the strengths and limitations of the study, and notes that it is difficult to make changes around screening mammography practices “because governments, research funders, scientists, and medical practitioners may have vested interests in continuing activities that are well established.”
Indeed, the American College of Radiology — which represents radiologists who perform and interpret mammograms — has issued a statement criticizing the study. This organization also objected to the 2009 recommendations on routine mammography, put forth by the U.S. Preventive Services Task Force, which we discussed here.
The National Partnership for Women and Families also provides a summary of the research, and Breast Cancer Action has issued a response as well, noting that the findings may not apply equally to all women:
The information that we have seen from this study does not let us draw conclusions for specific communities, many of which are most affected by higher morbidity rates. Given the unequal burden that women of color bear when it comes to breast cancer morbidity and mortality, questions remain as to whether there are different findings for difference communities. Absent this evidence, there are no clear answers for women of color. The growing body of evidence, of which this study is part, continues challenge long-standing and commonplace assertions that mammograms are a driving factor in reducing death rates.
What this research makes clear, however, is that we need more effective methods for detecting breast cancer, and new ways of determining which cancers need treatment.
Previous posts on the evidence about mammograms include:
- False Alarms Remain a Huge Problem with Mammograms Used for Breast Cancer Screening (Jan 2014)
- Do Screening Mammograms Do More Harm Than Good? (Dec 2012)
- The Benefits and Harms of Routine Mammograms (Dec 2012)
- Mammograms: How Effective Are They? (Nov 2011)
- New Mammogram Guidelines Are Causing Confusion, But Here’s Why They Make Sense (Nov 2009)