The excessive focus on mammography
Now that we’ve spent the month of October scaring women about breast cancer, isn’t it time for a month scaring them about mammography? If not a month, a week? A day?
It’s not right to scare people, but it’s also not right to leave them in the dark about an important debate going on in the medical community. While all agree that a mammogram is an important diagnostic test for women with new breast lumps, its use as a routine screening test is more contentious. That’s because screening mammography is a double-edged sword: It lowers the breast cancer death rate, but it also leads some women to be treated for cancer unnecessarily.
Let’s start with the good news. The U.S. Preventive Services Task Force -- independent experts who evaluate the effectiveness of health screening services -- says women who get mammograms every one to two years probably have a 16% to 19% lower death rate from breast cancer. That’s why they recommend it.
But they are also clear that it is by no means the most important thing we do in medicine. In fact, they estimate that about 1,000 women have to be screened for 14 years to avert one death from breast cancer. The other 999 don’t benefit. That’s why they don’t recommend it as strongly as screening for high blood pressure.
Now the bad news. While screening mammography probably reduces a woman’s chances of dying from breast cancer, it definitely increases her chances of getting diagnosed with breast cancer.
The problem is that screening mammography finds too many cancers. Among women in a specific age group, medical researchers know about how many will develop breast cancers that will ever grow to cause symptoms or death. But mammography finds more than this number -- published studies estimate between 5% to 30% more. These extra cancers will never grow or spread to cause health problems or death. Doctors call this problem over-diagnosis.
Because doctors don’t know which cancers will be harmful, we treat all of them. That means some women are needlessly given the diagnosis of “cancer” (itself terrifying) and needlessly undergo disfiguring surgery and the nausea, fatigue and hair loss associated with chemotherapy.
Over the last decade, medical researchers have begun to recognize that the problem of over-diagnosis is real. The uncertainty is no longer about whether it happens; instead, it’s about how often it happens.
Throughout Europe -- including Denmark, Italy, Norway, Sweden and Britain -- the initiation of population-wide screening mammography has been associated with substantial increases in the number of women diagnosed and treated for breast cancer. Although there was no single starting date for mammography in the U.S., there was about a 50% increase in the number of American women diagnosed with breast cancers associated with mammography’s introduction during the early 1970s to mid-1980s. One of the major randomized clinical trials of mammography reported 25 years of follow-up data in 2006 -- showing that about one in four cancers detected by mammography represent over-diagnosis.
But here’s some good news about the bad news: The harm of over-diagnosis is not a fixed attribute of screening mammography. Instead, it is related to how hard we look for breast cancer. Some women harbor small, innocuous breast cancers that will never cause symptoms or death (just as some men harbor small, innocuous prostate cancers). The harder we look, the more likely we are to find these cancers.
To understand the problem, use Google Earth to count the number of lakes in Utah. When viewing the entire continent, you’ll only see one -- Great Salt Lake. Now zoom in a bit. You’ll find two more -- Utah Lake next to Provo and Bear Lake on the Idaho border -- but they’re smaller. Now look even harder. Multiple lakes will suddenly appear high in the Uinta Mountains and the Wasatch Range. But they are smaller still.
The harder you look, the more lakes you find -- but they become smaller and less important.
“Look harder, find more” has been the prevailing paradigm in breast cancer screening from the outset. News reports focus on which approach finds more cancer. Conventional versus digital mammograms? Digital is better because it finds more cancer. Mammograms versus MRI? MRI is better because it finds more cancer. But the problem of over-diagnosis means that finding more cancer is not better -- it’s the wrong way to measure progress. Real progress would be to find only the cancers that matter.
Screening proponents fear that women can’t deal with the nuance. that mammography helps some and hurts others -- or any messages that might discourage women from having a mammogram every year. But leaving women in the dark exacerbates the problem of over-diagnosis.
How? Women who don’t know about over-diagnosis don’t question the “look harder, find more” paradigm -- and then all the forces line up to make the problem worse. Radiologists look harder at mammograms, pathologists look harder at biopsy specimens -- both only afraid of missing cancer, not over-diagnosis. Medical journals will continue to conclude (as will the news media that cover them) that the best test is always one that sees more, not less.
Women should be aware that looking harder may not be in their interest. And that doctors who recommend less-aggressive mammography (less frequently, waiting until you are age 50, or stopping it when you are older) or are less quick to biopsy may not be bad doctors but good ones. And women should demand (and participate in) research that looks less hard, finds less cancer -- but finds the cancers that matter.
H. Gilbert Welch is a researcher at the Department of Veterans Affairs and professor of medicine at the Dartmouth Institute and the Dartmouth Medical School. He is the author of “Should I Be Tested for Cancer? Maybe Not and Here’s Why.”