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And I'm Robert Siegel. Now, a new chapter in a long-running debate about the value of mammograms. Every year, American women get nearly 40 million of them; most of the time, to screen for unsuspected breast cancer. Well, a new analysis - out today, in "The New England Journal of Medicine" - looks at 30 years' worth of data. And as NPR's Richard Knox reports, it concludes that mammograms are leading thousands of women to get treated for breast cancer unnecessarily.

RICHARD KNOX, BYLINE: Dr. Gilbert Welch, of Dartmouth Medical School, says the purpose of the study was to get down to a very basic question: Do routine mammograms do what they're supposed to?

DR. GILBERT WELCH: For screening to work, it must take women who are destined to develop late-stage cancer, and find them when they're early-stage.

KNOX: So Welch and his colleague Dr. Archie Bleyer looked at what happened after mammography screening came into use, in the mid-1980s. They looked at how many women were diagnosed with early-stage breast cancer over the next three decades, and how many were diagnosed with late-stage cancer.

WELCH: And what we see is a dramatic increase - a doubling - in the amount of early-stage cancer. But we don't see a corresponding decrease in the amount of late-stage cancer.

KNOX: They say that means mammography isn't catching many advanced breast cancers, even though it's very good at catching early tumors. And moreover, they say many of the early tumors revealed by mammography, don't need to be treated at all. Doctors call that overdiagnosis. Welch says more than a million women have been overdiagnosed with breast cancer over the past 30 years, and the problem continues.

WELCH: Seventy thousand women a year are overdiagnosed, and treated unnecessarily, for breast cancer.

KNOX: This all may come as a shock to many women - and their doctors. No other diagnostic test has been more aggressively promoted than mammography; or lately, been so controversial.

DR. CAROL LEE: Whenever I see a paper like this, I say, oh, boy. Here we go again.

KNOX: That's Dr. Carol Lee. She's a mammography specialist at Memorial Sloan-Kettering Cancer Center, in New York City. She's also an official of the American College of Radiology, whose members do mammography. The college, in a statement, says the new analysis is simply wrong. Lee doesn't go quite that far, but she's highly skeptical.

LEE: I find it hard to believe that seven - what did they say? - 70,000 women in 2008 were diagnosed with breast cancer that would not have progressed? I just - I - I find that incredibly difficult to believe.

KNOX: Doctors will probably be arguing about the new study for some time. But Lee says the debate isn't very helpful to most women.

LEE: What my friends in Connecticut want to know is, should I have a screening mammogram? And I think this kind of study just really - sometimes raises more questions than it answers.

KNOX: San Francisco breast surgeon Laura Esserman agrees that women are tortured by these endless debates.

DR. LAURA ESSERMAN: They're caught in the middle. This is like two physicians fighting over how to treat a patient, and catching the patient in the middle. That's what's terrible. Shame on us.

KNOX: Esserman agrees that 20 to 30 percent of breast cancers diagnosed with mammograms, these days, are not life-threatening. But she sees a way out of this dilemma.

ESSERMAN: Our concept of cancer has got to change. We now recognize that there isn't just one pathway. It's not cancer - yes or no; and cancer that inexorably progresses to death.

KNOX: But highly sensitive mammograms pick up all kinds.

ESSERMAN: When you screen, you necessarily surface what we call idle lesions, or idle conditions. That is a consequence of screening. It doesn't make screening good or bad. So screening will be bad if we don't recognize these idle conditions, and we overtreat them.

KNOX: Esserman thinks mammography screening should be done more selectively. Women at lower risk of breast cancer - because they don't have close female relatives who've had it, or a genetic predisposition - may not need to be screened so often.

ESSERMAN: We're not going to screen our way into a cure. The most aggressive cancers show up in-between normal screens.

KNOX: Second, she says women and their doctors have to get out of the mindset that any breast cancer should be maximally treated. Currently, doctors routinely use surgery, hormones and even radiation to treat a condition called DCIS, ductal carcinoma in situ.

ESSERMAN: You know, we can watch a lot of those things. And most of those things turn out to be just fine.

KNOX: But this is a long way from how either mammography, or breast cancer treatment, is practiced right now.

Richard Knox, NPR News.

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