Although colleagues have from time to time wondered if I’m a witch (this started when I wrote a column saying the full moon is not associated with weird stuff, and then a few days later the 2004 Asian tsunami hit—during a full moon), I am definitely not psychic. So when I wrote the column in the current issue on screening for cancer, and the limitations of early cancer detection, I did not know that a study would appear yesterday evening with the enticing title, “What is the point: will screening mammography save my life?”
Unfortunately, it concludes that screening mammography (that is, giving mammograms to women with no symptoms of breast cancer and no elevated genetic risk for the disease) makes so little difference when it comes to cancer mortality—and really, what does anyone care about when it comes to cancer except not dying of it?—that the average benefit of a single screening mammogram is minuscule. For a woman of 40, the researchers calculate, the survival percentage is 99.52 percent if she does not undergo a screening mammogram and 99.62 percent if she does.
For their study in the open access journal BMC Medical Informatics and Decision Making, John D Keen of the John H. Stroger Jr. Hospital of Cook County and James E Keen of the University of Nebraska define “life saved” to mean that screening identified and helped cure one woman with breast cancer who would have died from the disease without screening. They then analyze U.S. data to estimate several things: the risk of dying from breast cancer if a woman does not undergo screening mammography, for instance, as well as the separate effects on breast-cancer mortality of screening mammography and improved therapy, and the survival percentages without and with screening. As you can tell, this is basically a mathematical analysis, not another clinical trial of mammography, and people will certainly argue with some of the assumptions the researchers used for their calculations (more on that below).
The numbers that popped out of the calculations were less than encouraging. In addition to the findings above, they report that “repeated screening starting at age 50 saves about 1.8 lives over 15 years for every 1000 women screened. . . . The average benefit of a single screening mammogram is 0.034%, or 2970 women must be screened once to save one life.” Overall, “Less than 5% of women with screen-detectable cancers have their lives saved” by screening mammography.
The mortality reduction with screening mammography is so low, in part, because for women under 65 the risk of dying of breast cancer is low to begin with: 1 percent over 15 years starting at age 55. With numbers that low, mammography would have to be highly selective and specific, and cure rates much higher with cancers detected early rather than late, to make a substantial difference in mortality. None of these is the case.
Last year, another study by the two scientists concluded that screening mammography has much greater life-saving benefits among older women. In the current study, that finding is replicated: the life-saving benefit of mammography increases with age. Repeated screening starting at age 50 saves about 1.8 lives over 15 years for every 1000 women screened.
The journal calls the study “highly controversial,” which is surely an understatement. For one thing, more than 90 percent of women believe that “early detection saves lives,” notes the journal. As I said in my column, people cling desperately to the notion that they can do something—namely, undergo cancer screening—to keep cancer from taking their life.
Because the study is likely to incite debate, the journal solicited two commentaries on it. In one, Stephen Duffy of Cancer Research UK, a British cancer-awareness group, argues that empirical data might be more trustworthy than the estimates the Keens derived by combining data from disparate sources. (A 2003 study that Duffy led found that mammograms did reduce breast cancer mortality.) He questions the accuracy of their numbers, noting that clinical trials have found that screening 1,000 women can prevent 2.1 (in a Swedish study) to 3 deaths from breast cancer (not the 1.8 the Keens calculate). And while the Keens calculate that fewer than 5 percent of women who undergo screening mammography have their lives saves, says Duffy, a Swedish study found that 141 breast cancer death were averted in a mammogram study that detected 928 cancers, for a mortality benefit of 15 percent.
In the opposing corner is Michael Retsky of Harvard Medical School, whose research has raised questions about the value of screening mammography, especially for women under 50. He writes in his commentary that the Keens’ study is on the right track in that women under 50 who undergo screening mammography face very real risks (of false positives and the resulting biopsy) for little benefit.
For another view on screening, however, I highly recommend the blog post of the American Cancer Society’s “Dr. Len”—J. Leonard Lichtenfeld, whose post on the PSA test I mentioned yesterday. Writing on the benefits of cancer screening in general, he argues that “comments that screening for cancer isn’t the answer to reducing deaths from every cancer are not new. Also untrue is the implication that those of us who advocate screening for certain cancers aren’t aware of the risks of screening. . . . Yes, we do believe that the scientific evidence shows that mammograms save lives. . . I remember what it was like to treat women with ‘early’ breast cancer in the days before effective mammograms were available, when a lump was palpated by the woman or her doctor and we told her it was ‘early’ cancer. Many of those women went on to die from their disease.”
As the debate continues, what we need is less fighting about whether screening mammograms save 5 percent or 15 percent of women who have them, and more progress on finding a screening method—paired with effective treatment—to radically boost those numbers.