Map of life expectancy at birth from Global Education Project.

Wednesday, March 29, 2006

A confusing decision, but one you need to make

The subject of screening tests comes up here a lot. There are a lot of these tests that doctors really like to do, and to most people, it seems obvious that there's nothing to lose by having a test that might catch cancer or some other serious condition early, while it's easier to treat. The story is not nearly so simple, but most of the time our doctors don't burden us with the complexities, they just tell us to get the mammogram or the Prostate Specific Antigen test, and we do it.

I've written in the past about Bayes' Theorem -- how even a highly specific test (one that only reads positive in a small percentage of people who don't have the disease) can be wrong most of the time when the underlying prevalence of a disease is low. But the issue with breast and prostate cancer screening is even trickier. The problem is, we aren't even sure what constitutes a false positive.

Some proportion of these cancers -- in the case of prostate cancer, we know for certain that it's actually a majority -- will never cause a problem. Either the person dies of something else before the cancer becomes clinically significant, or the cancer is what is called "indolent," that is it doesn't grow very much and it doesn't metastasize, it just sits there. Is that even cancer? It's a semantic quibble -- the problem is that we don't know how to tell them apart from the ones that will go on to cause trouble.

So if you're considering screening, you have to weigh the possible benefits -- that you might find a cancer early, when it can be effectively treated, that would eventually have killed you otherwise -- vs. the costs and risks, which include the possibility that you will end up having surgery, and/or radiation, and/or chemotherapy, with attendant risks, pain, high monetary cost, and in the case of mastectomy disfigurement, and in the case of prostatectomy incontinence and erectile dysfunction, plus the anxiety and general sturm und drang for yourself and your loved ones, all for no good reason at all.

A new study by Zackrisson, et al, published in BMJ, based on a randomized controlled trial of screening mammography done in Sweden in the 1970s, estimates the rate of overdiagnosis to be 10%. They got this number in a simplistic way: 15 years after the trial ended, there had been a total of 10% more cancers found in the screened group than in the unscreened group. The logic is that that many additional cancers must also have existed in the unscreened group but never caused a problem and so were never detected.

But as some letter writers point out, this is actually a considerable underestimate of overdiagnosis. Gilbert Welch, Lisa Schwartz and Steven Woloshin of Dartmouth note that real issue is the percentage of cancers found by screening that represent overdiagnosis. Since some cancers in the screened group were not found by screening but by clinical diagnosis; and as cancers in both groups continued to accumulate after the trial ended 15 years ago, the percentage of positive mammograms that represent what they call "pseudocancer" was 24%, not 10%. Peter Gotsche points out that some of the women in the control group actually got screening mammograms after the trial ended, which pushes the rate of overdiagnosis up even higher. He thinks at least 30% is the right number.

Now that doesn't mean you shouldn't get a screening mammogram. Important additional considerations include your personal risk factors (e.g., if your mother or sister has had breast cancer, or you haven't had children, you might be more inclined to be screened), and how averse you feel personally to the treatments for breast cancer, as well as your age and your philosophy of life. There is reasonably good evidence that in the long run, screening does reduce the breast cancer death rate in a population, so all things being equal it might give you a chance of living longer -- but it's a small chance.

What I advocate for is not screening or not screening, but knowledge, and autonomy. Doctors should inform women much more fully about these issues than they typically do. Rather than blindly following recommendations from such bodies as the American Cancer Society (which have a vested interest in promoting treatment since they are closely allied with drug companies, surgeons, radiologists and oncologists), women who are so inclined should make up their own minds.

I'll get to prostate cancer screening another time.

No comments: