Getting to the Bottom of the Mammogram Debate

Music, we’re told, can soothe the savage breast. Data about breasts, in contrast, can ignite rather savage controversy, and propagate confusion.

That seems to be the immediate result of a newly reported study that found a 26% mortality reduction with routine mammography for women in their 40s. This would just be good news, if prior studies had not failed to confirm just such a benefit- and if the US Preventive Services Task Force had not recently weighed in against routine screening for women under age 50.

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The new study, conducted in Sweden, involved roughly a million women.   Combine a large study; high profile media coverage; a politically charged topic; and a large reservoir of passion, and you have a very combustible mix indeed!   Let’s put out the fire.

First, the new study looked at the benefits of mammography only, not its harms. It took advantage of a natural experiment in Sweden: some counties offered mammograms routinely, others did not.   Mortality was 26% less overall in those counties that did.

You should immediately think to ask: 26% lower than what?   Breast cancer is less common among women under 50 to start with.   The flip side of the risk/benefit equation is reflected in the fact that well over 1,000 women under 40 need to be screened for ten years before screening saves one life.   Perhaps still worth it- but 1,000 women screened for ten years, with all of the resultant false positive tests and biopsies that ensue from that- is not a trivial price to pay to find that one case that really needed to be found.

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We also don’t know from the Swedish study what else may have differed between counties that did and counties that didn’t screen.   Maybe those that did screen also provided better care- and the apparent survival benefit was more about treatment than screening.   Maybe the populations differed in other important ways.

There are many reasons why mammography before 50 is controversial while mammography after 50 is not.   For starters, breast cancer is substantially more common after menopause than before.   Screening is of greatest value when what you are looking for is neither too common, nor too rare.   When breast cancer is relatively rare, screening produces many false positives for every true one.

Second, younger women tend to have denser breasts, making mammograms harder to interpret- and increasing the risks of both missing cancer, and false positives.   Third, breast cancer in women under 50 often grows faster- reducing the benefits of screening.   It might be that the best screening for younger women would be twice a year, rather than once- although that, in turn, might increase the harms of radiation exposure.   We await the studies that would answer this question

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When the new study is scrutinized, and compared against what we thought we knew before- there is more smoke here than fire.   The official recommendation in the US was neither for, nor against, mammography in women under 40; rather, it was for individualized decision making because the risk/benefit trade-off is a very close call.   For a given woman, the benefits of mammography may clearly outweigh the risks; but for others, they may not.   You and your doctor can review your risk factors, and decide. If you decide to be screened, your insurance company should certainly pay.

The new study is certainly noteworthy — but it really shouldn’t start a fire.   The pros and cons of mammography in younger women, just about evenly balanced, are still simmering as they ever were.     Let them, and your passions on the topic, cool- and confer with your doctor about the right strategy for you.   The news may be new, but your options are just as they were this time last week

by David Katz, MD, PREVENTION, on Thu Sep 30, 2010 11:44am PDT


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