January 25, 2009


Risk factors determined by medical stats (Judy Peres, January 25, 2009, Chicago Tribune)

Schwartz and Woloshin recalled an old magazine ad that proclaimed, "If you are over 35 and haven't had a mammogram, you need more than your breasts examined."

The obvious implication was that it's crazy not to get tested for breast cancer. But that ad failed to mention that, for a 35-year-old woman, the chance of dying from breast cancer over the next 10 years is only about 0.1 percent (or 1 in 1,000). And there is no evidence that mammograms save lives for women under 40.

A more recent newspaper ad from a respected cancer center carries this message: "The early warning signs of colon cancer: You feel great. You have a healthy appetite. You're only 50."

The ad—an attempt to scare readers into getting tested for colon cancer—suggests you might have the disease without knowing it. But how likely is that? And what benefit would you derive from getting tested?

In a new book, "Know Your Chances: Understanding Health Statistics," Schwartz, Woloshin and Dr. H. Gilbert Welch walk readers through the calculus.

Using the example of colon cancer screening, they suggest the first question to ask is, "What is my risk of getting colon cancer?"

You may have read that "colon cancer will strike about 150,000 Americans," which sounds like a lot. But there are 300 million Americans, and 150,000 out of 300 million is 0.05 percent, which seems a little less ominous.

If you believe the risk is worth thinking about, the next question would be, "What's the benefit of getting tested?" Clearly, a screening test will not protect you from getting colon cancer. But because of early detection, it might keep you from dying of the disease.

So, "What's my risk of dying of colon cancer?" The authors favor looking at risk in 10-year chunks because a decade is short enough to imagine but long enough to make changes that might reduce the risk. (Many organizations, including the American Cancer Society, speak of lifetime risk—the chance of something happening anytime between birth and death. That leads to relatively impressive statistics, such as "One in eight women will get breast cancer.")

Using the charts in the book, you can discover that a 50-year-old man, for example, has a 2 in 1,000 chance of dying of colon cancer over the next 10 years.

Which leads to the hard question, "Is that a risk I should try to reduce?"

One way to answer that is to compare your risk of dying of colon cancer with your risk of dying of other things. That 50-year-old man with a 2-in-1,000 risk of dying of colon cancer can compare that to his risk of dying of lung cancer (18 in 1,000 if he's a smoker) or heart disease (29 in 1,000 if he smokes and 11 in 1,000 even if he never smoked).

Deciding to stop smoking or go on a diet might be a reasonable approach for someone with limited time and money to spend on health.

Schwartz and Woloshin suggest that another way to answer that question is to consider the absolute benefit of the proposed intervention. Clinical trials have shown that screening can reduce colon cancer deaths by as much as one-third. That translates into lowering the 50-year-old man's chance of dying from 2 in 1,000 to 1.3 in 1,000—an absolute difference of 0.7 per 1,000.

...whether the particular hysteria over breast and colon cancer isn't just driven by the fact that they're naughty bits. It's not ioncology, it's fetishism.

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Posted by Orrin Judd at January 25, 2009 8:30 AM
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