It is frequently alleged that American’s don’t understand math, and our constant failure to grasp health related statistics is usually held up as a primary example. I am of the belief that it is not our ability to grasp the math that is at fault, but the inability to accurately present the data that gets the public in trouble. Take, for example, the hysterical (and somewhat politically motivated) backlash against the new breast cancer screening recommendations. Even as someone who is deliberately and delightfully unaware of the political BS that surrounds me, I couldn’t help but hear misty-eyed women on the radio claim that “I am not a number” and “the mammogram saved my life”. (This is why I hate politics – in politics one individual’s anecdotal evidence carry more weight than a scientific study thousands of times larger). Nowhere however did I hear a single piece of science or math. The rate of cancer in the population, the false positive and false negative rate of mammograms, and other basic facts where all missing from the reporting. When I set about to find these number on the Internet a few hours ago, it took a shockingly long time because almost no media source bothered to cover it. (Instead I got lot of emotions – relief, anger, shock, ecstasy, disbelief, sadness, etc.). What follows is my attempt to do what the media should have – explain why this decision was made using that little thing we like to call “evidence”.

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The old advice for women was to begin getting annual screening mammograms (regardless of personal risk) at 40. Since the new advice is to start at 50, I will be focusing on women in the 40-50 range. The odds of a women in her 40s getting breast cancer are 1 in 69, or 1.45%.¹ (This number includes those in high risk groups). Mammograms, as I found out, are shockingly inaccurate. False positive rates over the course of 10 mammograms (so, a women who followed the old advice in her 40s), has been shown to be 56%.² To put that shockingly bad number in perspective, flipping a coin once yields only a 50% false positive rate. So what does this tell us – if 69 women get mammograms over their 40s, one of them will develop cancer. Of the other 68, 38 of them will have something detected on a mammogram, despite not having cancer. (68 * .56 = 38.08). That means if you have something detected on a mammogram in your 40s, you still only have a 1/39 chance of actually having cancer. This is why, in a recent UK trial showed “there was a reduction in breast-cancer mortality… which did not reach statistical significance.”³

Some of you are probably already convinced once you realize that a positive mammogram in your 40s still means you have only a 3% chance of having cancer. However there are a lot of people in this country who seem to think that an infinite number of medical tests for everyone is a good idea with no repercussions. While I cannot find the mortality rate for unnecessary surgery or a biopsy, suffice it to say that they are greater than zero. In a 10 year retrospective study, 631 mammogram false positives resulted in 128 unnecessary biopsies and 1 hospitalization for complications during the unnecessary biopsy.² All procedures have risks, and increasing the number of unnecessary procedures also increases the amount of harm that they will cause. Studies have also shown incredibly high levels of anxiety, morbidity, and other forms of mental distress stemming from false positive mammograms.^1,2 One study even showed that 17% of women with a false positive mammogram reported it affected their daily activities months after the fact.⁴

Lest I accidentally convince you that mammograms are completely useless, you should know that mammograms become more effective with age as the breasts age and become less dense. Also since age is still the number one indicator in whether or not a woman will develop breast cancer, their usefulness does increase with age.⁵ This is why the new advice is still to have regular screenings from age 50 and onward.

Sources:

1. Screening for Breast Cancer: An Update for the U.S. Preventive Services Task Force, Agency for Health Research Quality, November 2009
2. Elmore JG, Barton MB, Moceri VM, Polk S, Arena PJ, Fletcher SW. Ten-year risk of false positive screening mammograms and clinical breast examinations. N Engl J Med. 1998;338:1089-96.
3. Moss SM, Cuckle H, Evans A, Johns L, Waller M, Bobrow L; Trial Management Group. Effect of mammographic screening from age 40 years on breast cancer mortality at 10 years’ follow-up: a randomised controlled trial. Lancet 2006;368:2053-60.
4. Lerman C, Trock B, Rimer BK, Boyce A, Jepson C, Engstrom PF. Psychological and behavioral implications of abnormal mammograms. Annals of Internal Medicine 1991;114:657-661.
5. Adams, Jill U., Getting to the facts in the debate on mammograms, The Los Angeles Times, Nov 23, 2009

Now tell me honestly – does treating you like an adult and showing you the numbers make you more or less inclined to listen to the new guidance?

This entry was posted on Wednesday, December 2nd, 2009 at 4:25 pm by Ari and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.