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Scientists say they might have solved a big medical mystery - why mammograms don't save more lives. The report appears in The New England Journal of Medicine. NPR health correspondent Rob Stein has more.
ROB STEIN, BYLINE: When it comes to breast cancer, Donald Lannin at Yale says there's been a big assumption for a long time.
DONALD LANNIN: For over 100 years, we've known that small breast cancers have a much better prognosis than large breast cancers. And we always assumed that it was because we were catching the small cancers early and that that's why the cure rate was much better.
STEIN: But Lannin wanted to find out if that was really true, so he analyzed thousands of breast cancer cases for more than a decade. And what he found was surprising.
LANNIN: We thought almost all breast cancers were alike. You know, breast cancer was breast cancer. It was bad. And what we're finding is that breast cancer varies tremendously from some cancers that are extremely fast-growing and aggressive to other cancers that are extremely slow-growing.
STEIN: And about 1 out of every 5 tumors that mammograms find aren't the fast-growing bad kind. They're the good ones. They're tiny because they grow very slowly.
LANNIN: One of the main reasons small breast cancers have a better prognosis than large breast cancers is because they're biologically much more favorable and much slower-growing. It takes 15 or 20 years for them to cause any problems.
STEIN: That means for many women, the tumors would never become life-threatening.
LANNIN: You can kind of imagine that a lot of patients will die of something else over that 15 or 20 years.
STEIN: So, Lannin says, a lot of women who get screened for breast cancer with a mammogram end up going through a lot of miserable stuff - biopsies, chemo, radiation, mastectomies - unnecessarily.
H GILBERT WELCH: That means that screening disproportionately finds good cancers, cancers that may be better off not found.
STEIN: That's H. Gilbert Welch. I reached him at Dartmouth over Skype.
WELCH: I think we all need to realize that we probably oversold the idea that looking for cancer early is the best way to avoid it. Mammography is a really close call. It's a choice. We've exaggerated its benefit and we've sort of understated its harms.
STEIN: But not everyone agrees. Debra Monticciolo is a radiologist at Texas A&M Health Science Center. She says the new study has lots of flaws.
DEBRA MONTICCIOLO: We know that mammograms have led to finding tumors earlier and have decreased mortality overall and it's pretty significant. And what I'm concerned about is that this will give patients or their physicians the idea that, well, you know, if you have a tumor with a, quote, unquote, "favorable" biology that, gee, you know, we just don't need to worry about it. And that's just not true.
STEIN: Others argue that the long-term solution is to come up with new tests, tests that can tell the difference between tumors that are more likely to grow fast and become life-threatening and those little ones that may really be nothing to worry about. Otis Brawley is the chief medical officer at the American Cancer Society.
OTIS BRAWLEY: What we need to do is go from a mid-19th century definition of cancer that involved a biopsy to a 21st century definition of cancer that involves both a biopsy and genomic testing so that we can say, Mrs. Jones, you have a breast cancer that we should watch or Mrs. Smith, you have a breast cancer that we need to treat.
STEIN: In the meantime, Brawley recommends women continue to follow their doctor's advice about getting mammograms. Rob Stein, NPR News.
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