Up at 5AM: The 5AM Solutions Blog

Mammography Screening - Don't Panic!

Posted on Mon, Dec 14, 2009 @ 01:01 PM

Surely you saw the newspaper articles about the recent government guidelines about breast cancer screening using mammography. There were breathless articles and letters to editors complaining about how mammograms had saved lives and how could a heartless government agency say we shouldn't continue with the practice. Congressional hearings were held with "debate split along party lines" so you know they really got down to the science of it.

The main idea that caused so much consternation was that women aged 40-49 should not routinely undergo mammograms to look for signs of breast cancer, which is what previous guidelines had said. But let's look at what these new guidelines really say:
  • "The USPSTF recommends biennial screening mammography for women aged 50 to 74 years"
  • "The decision to start regular, biennial screening mammography before the age of 50 years should be an individual one and take patient context into account, including the patient's values regarding specific benefits and harms"

So it doesn't say that women in their 40's should not get mammograms. It says they should weigh the risks and benefits themselves (and with their doctor, obviously, although I wish it had said that explicitly) and make their own decision.

So why not get mammograms earlier? The term 'screening' is key here. A screening test is a test that is given when there's no prior evidence or risk for a condition. A cholesterol test is a screening test, for instance, to look for evidence of heart problems.

You have to remember that by its definition a screening test is given to large number of people, only a small fraction of which have the condition being tested for. So even if such a test is relatively accurate, there will be a large number of false positive results. In the case of mammography, a false positive (and I hesitate to use the word 'positive' here) is an abnormal result when in fact the patient does not have cancer. There's a useful statistic to quantify this called positive predictive value (PPV). PPV is the fraction of positive results that actually have the condition being tested for. For women aged 40-49, mammography has a PPV of 2-4%. So that means that only a small percentage of women in that age group who have an abnormal mammogram actually have cancer.

The critics of these guidelines have said this is fine since we want to catch as many cancers as early as possible. But you have to take into account that no test is without risk, and that more invasive procedures, such as biopsies, are done when a mammogram is abnormal. The mammogram itself, as well as the subsequent procedures, have a monetary cost as well.

What the guidelines say is that women under 50 should get mammograms if they want to, or if they or their doctor feels there are other reasons for them to have a high risk of breast cancer. If one has a family history of breast cancer, or a genetic risk factor, then those would be good reasons to have earlier mammograms, in my non-medical opinion.

So this is a case of reasonable scientific conclusions being misinterpreted. However, I do think there needs to be more research done to say how many cases of cancer would be missed if mammograms were only done for women aged 40-49 who had some other risk factor. That would make it more concrete for women about what risk they were running by not having mammograms before age 50.


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