Writing in the L.A. Times yesterday, freelance health writer Christie Aschwanden gives a succinct summary of major mammography studies published since the U.S. Preventive Services Task Force issued its highly controversial recommendations in November 2009 against routine mammography screening for women in their 40s and in favor of every-other-year screening for older women.
As Aschwanden points out, the findings from these latest studies do not trump the task force’s recommendations, despite the studies’ somewhat contradictory results. It all has to do with the quality of the studies’ methodology:
A study published last March in the journal BMJ compared women who lived in a region of Denmark where mammography screening was offered to those who lived in areas without screening and found no reduction in breast cancer deaths associated with mammography.
A similar study published in the journal Cancer in September compared breast cancer death rates in women from a region of Sweden with a public mammography program to those in an area without the program and found that deaths were 29% lower in the area with a screening program.
Yet another study that month, published in the New England Journal of Medicine, compared breast cancer deaths in women taking part in a Norwegian national screening program with those who were not screened, looking at mortality rates in those areas before and after the screening program began. This study found that breast cancer deaths had dropped since the mammography program began. But it calculated that most of the improvement was attributable to increased breast cancer awareness, which led women to seek treatment right away for any lumps or bumps they discovered, and new treatments. The contribution of routine mammography to the reduction may have been as small as 2%.
How could these studies yield such contradictory results? The answer is simple, says Dr. Heidi Nelson of Oregon Health and Science University in Portland, who led a research team that compiled evidence for the [U.S. Preventive Services Task Force]. All of these studies were observational, meaning they were done on groups of women who either chose to get mammograms or didn’t — and there can be very real differences between those two groups.
“If you just look at the women who show up for mammography, you’re looking at a biased sample of people,” Nelson says: Women who choose to get mammograms are usually healthier and more health-conscious than those who skip the screenings. For this reason, the task force based its recommendations on the evidence from trials that took groups of women who were as similar as possible and randomly assigned them to either a screening or no-screening group.
“We had eight randomized trials. That’s as good as it gets in our line of research,” Nelson says.
When the researchers looked at these eight trials individually, none of them showed a benefit from mammography, Nelson says. It was only when the trial data were pooled that a benefit emerged — a 15% reduction in the risk of dying of breast cancer. Though that sounds “pretty respectable,” she says, it’s actually quite modest because the number of women who die of breast cancer in their 40s is tiny.
The task force recommended that women discuss the risks and benefits of mammography screening with their physician and then make the decision that seems best for them. (This point was often lost in the media hysteria that followed the publication of the recommendations.) It’s unclear, though, how many women are having that discussion.
As Aschwanden points out, an online survey conducted by a medical journal last year found that more than 70 percent of the 241 women who responded to the survey said they would continue to get an annual mammogram during their 40s — even if their doctor recommended against it. (A caveat: This was an “opt-in” survey, which runs a great risk of selection bias in favor of people who are most passionate about an issue.)
Interestingly, though, half of the 345 physicians who responded to another online survey by the same journal reported said they had changed their guidance to patients about mammography screening. Some 67 percent reported that they no longer offered routine mammograms to women in their 40s, and 62 percent said they now advised women aged 50 to 74 to be screened every other year rather than annually.
You can read Aschwanden’s article here.