But screening poses another downside: A routine mammogram can find cancers that would never have become life-threatening, subjecting women to painful and toxic treatments they never actually needed.
A new study calculates that this is just what happens in as many as one in every three breast cancers diagnosed by a screening mammogram. That research, published July 9 in the British Medical Journal, comes on the heels of several other studies suggesting that some breast cancers found on mammograms would naturally have regressed on their own without treatment.
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The studies don't mean that women should abandon mammography, most experts say. But some think it's time to reconsider the way that mammogram screening is done.
"For too long, we've taken a brain-dead approach that says the best test is the one that finds the most cancers -- but that's wrong," says Dr. H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, N.H, who wrote an editorial accompanying the British Medical Journal study. "The best test is the one that finds the right cancers and nothing else."
The detection of tumors that would never have caused trouble is known in the medical trade as overdiagnosis. It's a common problem with screening tests, which, by definition, aim to detect disease in people without symptoms.
Breast cancers generally behave in one of three ways. Some grow very aggressively and metastasize (i.e. spread to other tissues) long before any mammogram can detect them. Others grow more gradually and can be successfully treated if caught early. Still others grow so slowly that they'll never cause the woman a problem.
Autopsy studies have found undetected breast cancer in about 37% of women who died of some other cause. And a study of 42,238 Norwegian women published in November calculated that 22% of symptom-free cancers found on a screening mammogram naturally regressed on their own.
The problem is that, even under a microscope, it's impossible to distinguish these different types of cancer from one another, and mammograms are better at catching the less dangerous kinds.
"Screening is really good at finding the slow, ploddy, probably-not-going-to-kill-you cancers, but it's not so good at finding the fast, aggressive ones," says Susan Love, president of the Dr. Susan Love Research Foundation and author of "Dr. Susan Love's Breast Book."
The July 9 study, by researchers at the Nordic Cochrane Centre in Copenhagen, examined rates of breast cancer in regions of the United Kingdom, Canada, Australia, Sweden and Norway before and after these countries instituted national mammography programs.
The scientists reasoned that if screening mammography were preventing early-stage breast cancers from progressing, these programs should have resulted in a drop in the number of advanced breast cancer cases -- those showing signs of spreading -- among women who had been screened.
The researchers didn't find that. Instead, they found that mammography screening programs increased the overall number of breast cancers diagnosed but did not reduce the number of advanced cancers.
The team calculated, based on these results, that for every 2,000 women screened by mammography over 10 years, one will avoid dying from breast cancer and 10 others will receive treatments for a cancer that would have never become life-threatening.
In other words, "screening causes 10 times as many women to become cancer patients unnecessarily as it prevents from dying from breast cancer," says lead author Karsten Jorgensen, a researcher at the Nordic Cochrane Centre in Copenhagen.
To screen or not?
Because scientists do not yet have a way to distinguish cancers that will turn life-threatening from the harmless ones, they must treat every case as if it were the worst kind. As a result, Jorgensen says, for every woman whose life is saved, several others are subjected to surgery, radiation therapy and sometimes chemotherapy that they didn't need, for cancers they never would have known about without the screening.
"The question is no longer whether overdiagnosis occurs, but how should we react to it," Welch says. The recent studies suggest that "it's not an imperative to be screened; in fact, it's a close call."
Breast cancer survivor Barbara Brenner, executive director of Breast Cancer Action, a breast cancer advocacy group in San Francisco, says it's time to address this problem.
"Every woman who had a breast cancer found on a mammogram thinks the mammogram saved her life," she says. "There are probably some women whose treatments have led to their early deaths. But no one wants to talk about that."
But Elizabeth Thompson, vice president of health sciences at Susan G. Komen for the Cure, a breast cancer advocacy group based in Dallas, says she worries that these studies will undermine her group's awareness efforts.
"I don't think you can say that we're overtreating those women. We know that some of these cancers become invasive," she says. "We need to keep hammering away at our basic message, which is, early detection saves lives."
Dr. Eric Winer, director of the breast oncology center at the Dana-Farber Cancer Institute and chief scientific advisor for Susan G. Komen for the Cure acknowledges that messages about mammography may need revamping.
"As painful as it is to admit, we have oversold mammography to the American public," he says. "Frankly, I don't know what to do with this. On the one hand, I don't want to push people away from mammography, but I don't want to encourage them to have misconceptions about mammograms either."
Winer says it might be worth studying the effects of doing the screens less frequently. The U.S. is the only country that recommends yearly mammograms starting at age 40. Most European countries begin mammography at age 50 and recommend them only every two or three years.
Welch thinks it's time scientists figured out ways to make mammograms more effective.
"We need to reevaluate what we call abnormal on a mammogram," he says. Instead of looking so hard for very early cancers, doctors should focus on finding the ones most likely to turn deadly. Perhaps, he says, some minor abnormalities could be ignored, in particular small microcalcifications, miniature specks of calcium that are usually harmless but occasionally occur in tandem with precancerous changes in the breast.
But radiologists are unlikely to adopt guidelines that call for a wait-and-see approach, says Dr. Fred Vernacchia, a radiologist and medical director at the San Luis Diagnostic Center in San Luis Obispo. Mammograms are the No. 1 source of malpractice lawsuits against radiologists, he says, and this creates an incentive to overdiagnose breast cancer.
"The choice is, overbiopsy and overdiagnose -- or live through another lawsuit," he says.
The conclusion that many breast cancer patients never needed treatment may seem disheartening, but there is good news here also, Love says.
"What this says is that there's such a thing as cancer without disease. It means you don't have to just blast away every cancer cell," she says. Now, she adds, scientists need to figure out ways to tell which cancers are which and what factors in the breast might make dormant cancer cells wake up and start causing trouble.
"We shouldn't stop doing mammograms, because we have no alternative," she says. "But women need to walk away from this study recognizing that things are not as simple as we thought. We used to think that all cancers were the same and that early detection was prevention.
"Well, it's not prevention. And early detection may not always be in your best interest."