Does screening mammography save lives?

Dr. Pearlman also points to other studies of screening mammography that have ­produced findings contrasting those of the Canadian trial.

“At least eight large observational trials, case-control studies, and randomized, controlled trials of screening mammography have been published and were later evaluated by meta-analysis.⁸ That analysis showed a 50% reduction in mortality in women who had screening mammography. In both randomized, controlled trials, it showed a decrease of about 15% in mortality. In practice, looking at large populations of women who died of breast cancer and comparing them to women who had breast cancer but didn’t die, there is a 50% increased likelihood of dying if you don’t have screening mammography. So looking in both directions—both prospectively and retrospectively—there appears to be a substantial benefit to undergoing routine screening mammography in reducing breast cancer mortality,” Dr. Pearlman says.

Dr. Dickinson asserts that criticisms of the Canadian National Breast Screening Study were disproved long ago.

“Many of those accusations were brought out very early in the course of the Canadian trial and investigated in great detail and rejected. After all, this trial was funded by a major research funding body in Canada. And when it was informed that it had funded a ‘fraudulent’ trial, it investigated and found that the findings actually were legitimate,” says Dr. Dickinson.

“I think that the people who are still bringing up those accusations are doing it primarily because the results don’t fit what they wanted. It’s attacking the messenger because they don’t like the results.”

WEIGHING BENEFITS AND HARMS
When the Canadian Task Force on Preventive Health Care formulated its guidelines on screening mammography, it considered the same body of evidence assessed by the USPSTF for its 2009 guidelines. Dr. Dickinson, a member of the Canadian Task Force, notes that the Canadian approach differed from the American approach in several distinct areas.

“We used the USPSTF literature search up to 2008 and then we did an updated search, looking for papers published up to that time. But there were no new trials published from 2008 to 2011,” he says.

“So we looked at the same data but used the GRADE scheme, which carefully separates the strength of the evidence from the strength of the recommendations. It’s a ‘newish’ way of evaluating evidence,” Dr. Dickinson says. “It’s different from the USPSTF approach, which involves a different scale.”

“We used to assess preventive measures purely on the basis of efficacy—if they worked, we’d recommend them. Now we look at the balance of benefits and the potential for causing harm. So it’s not just about whether an intervention works, but about whether it works more than it causes harm,” he says.

“That means that you can have statistically significant benefits that are fairly small and are outweighed by harms. So, while screening mammography can significantly reduce the risk of death from breast cancer by a small amount, our recommendation for it is very weak because, to achieve that benefit, you also incur a lot of harm,” Dr. Dickinson says.

Dr. Pearlman agrees that “mammography is not a perfect test, by any means.”

“It’s inconvenient, people get worried, it’s uncomfortable, and it isn’t perfectly sensitive,” he says. “It’s also somewhat nonspecific, which means that about 10% of women who don’t have breast cancer will be called back for additional images, and about 10% of that group will get called back for a biopsy that is not due to cancer.”

HOW WE COUNSEL OUR PATIENTS
Dr. Kaunitz says he is less likely to recommend annual mammography screening in the wake of the Canadian trial and other findings.

“For decades, we have marched to the drumbeat of ‘mammograms save lives,’” he says. “Annual screens have become an easy recommendation for us to make and, for our patients, the reassurance that accompanies a normal mammogram is comforting. Many patients will be perplexed by this new information; others may view it with suspicion. While we await updated guidance from professional societies, my approach is to encourage patients to follow the 2009 USPSTF guidelines, which recommend that screening start at age 50 in average-risk women and be repeated every 2 years.”

Related articles:
Biennial vs annual mammograpy: How I manage my patients Andrew M. Kaunitz, MD (Commentary, June 2013)
Best age to begin screening mammograms: How I manage my patients Andrew M. Kaunitz, MD (Commentary, November 2013)

Dr. Dickinson takes a similar approach. “I recommend that people be cautious about having screening, but I listen to their stories. Someone may say, ‘My sister had breast cancer and I want a mammogram.’ Overall, I don’t encourage people to undergo mammography unless they have a strong reason for doing so. I try to follow the latest [Canadian] guidelines because I feel they’re based on the best available evidence.”