A sweeping U.S. study published on Nov. 22, 2012 in the New England Journal of Medicine shows that mammograms have done surprisingly little to catch deadly cancers before they spread. At the same time, they have led more than a million women to be treated for growths that never would have threatened their lives.
Women over 40 are routinely advised to have yearly mammograms, and it’s widely believed that having one is key to protecting a woman’s health.
Although experts agree that diagnostic mammograms are beneficial (cases where there is a breast lump or other symptoms), there is much controversy about screening mammograms, which are performed on women with no signs of cancer. Mammograms detect breast cancer, although many people believe mistakenly that they prevent breast cancer. We now know that the mortality benefits remain quite small.
Eight trials performed in the United States, Canada and Europe have evaluated the ability of screening mammograms to decrease the death rate from breast cancer, as well as overall mortality. Looking at the overall death rate, not just death from breast cancer, is essential, because this approach also evaluates whether the screening test and any subsequent treatment may be causing other harms.
Overall, the early studies showed a 30 percent reduction in the risk of dying from breast cancer in women who were screened by mammography. In 2001, a critical review of all eight trials by the Cochrane Collaboration found that six of them were sufficiently flawed to invalidate their results. The Cochrane Collaboration then pooled the results of the two remaining studies and found no evidence to support the use of screening mammography.
The U.S. Preventive Services Task Force (USPSTF) evaluated the trials also. Although recognizing many of the same flaws, the USPSTF felt only one trial was sufficiently flawed to be invalidated. They pooled the results of the remaining trials and found a 16 percent reduction in the risk of dying of breast cancer in the women in the screened group.
The meta-analysis published in 2006 by the Cochrane Collaboration confirmed that screening does slightly reduce breast cancer mortality, but that it also leads to over-diagnosis and overtreatment of breast cancer. They concluded:
(F)or every 2000 women invited for screening throughout 10 years, one will have her life prolonged. In addition, 10 healthy women, who would not have been diagnosed if there had not been screening, will be diagnosed as breast cancer patients and will be treated unnecessarily. It is thus not clear whether screening does more good than harm.
In a recent issue of the New England Journal of Medicine, Dr. Archie Bleyer and Dr. Gilbert Welch summarized the latest analyses as follows:
Despite substantial increases in the number of cases of early-stage breast cancer detected, screening mammography has only marginally reduced the rate at which women present with advanced cancer.
And this is the key to meaningful breast cancer screening — that we reduce the rate at which women have to be treated for late stage cancer.
When the data for women under 40 were studied (these are women who generally don’t get regular mammograms), Dr. Welch and Bleyer wrote:
There was a larger relative reduction in mortality among women who were not exposed to screening mammography than among those who were exposed. We are left to conclude, as others have, that the good news in breast cancer — decreasing mortality — must largely be the result of improved treatment, not screening.
Dr. Susan Love, a long time clinician and researcher, would like to see less emphasis on screening and more focus on cancer prevention and treatment for the most aggressive cancers. (Roughly 15 percent to 20 percent of breast cancers are deadly.)
“There are still 40,000 women dying every year,” Dr. Love notes. “Even with screening, the bad cancers are still bad.”
As Donald Berry, a biostatistician at M.D. Anderson Cancer Center in Houston, has pointed out:
Most breast cancers are not lethal, however found. Screening mammograms preferentially find cancers that are slowly growing, and those are the ones that are seldom deadly. Getting something noxious out of the body as soon as possible leads women to think screening saved their lives. That is most unlikely.
The challenge now is to make more widespread the use of techniques that help clinicians identify biological markers that will distinguish between the lethal and benign types of tumors. This appears to be the next big advance in reducing mortality from breast cancer.
Mammography, like other detection tools, is imperfect (it misses about 20 percent of lumps due to dense breasts and other factors). Some would consider it a very weak detection tool, and given the harms of overtreatment (for example, unnecessary chemotherapy and radiation treatments), it is not surprising that some women will want to forego screening mammography.
Women need to carefully consider these factors and decide for themselves what would be best, although friends, caregivers, and even commercial interests may tell them that having routine mammograms is the only rational choice. What really helps is knowledgeable and supportive counseling.
This article was originally posted at Cognoscenti, WBUR Boston’s ideas and opinions section, and is re-posted with permission.