Friday, December 02, 2011
If only science was a sure thing
Science is an uncertain science. That’s been brought home once more this past week with all the consternation over mammography.
“The Screening Mammography Program Saves Lives,” says the headline on the B.C. Cancer Agency’s on-line writeup about mammography, a type of x-ray of the breast that up until days ago was routinely promoted to Canadian women 40 and up as an annual must-have.
But the Canadian Task Force on Preventive Health Care has rocked the boat big-time with new recommendations that reduce the use of mammography.
The task force has toned down Canada’s 10-year-old guidelines around when to get mammograms. The revised guidelines suggest routine mammograms only for women ages 50 to 74 and even then no more than once every two or three years.
No big deal in the grand scheme of things. One less medical appointment to schedule.
But it’s disturbing when something that has been sold to us as an absolutely essential health measure suddenly reveals a dark side. The about-face on mammograms serves as an excellent reminder that health care can hurt.
In the case of mammograms, the issue is “false positives.” Mammograms are prone to turning up slow-growing lumps in the breast that look like cancer but in fact do no harm over a lifetime.
That means you can end up having surgery, radiation and chemotherapy you didn’t need - treatments that can damage your health permanently and waste precious health-care dollars to boot. False positives have been a major issue in prostate-cancer screening for years now for those very reasons.
Mammograms provide “a real benefit,” said task force chair Dr. Marcello Tonelli in media reports this week on the revised guidelines. “But compared with the risk of false positives, it’s relatively small. If you look at the numbers, you are much more likely to have a false positive result than you are to have your life saved with screening.”
New Yorker writer Malcolm Gladwell saw this one coming years ago. He wrote a brilliant article back in 2004, “The Picture Problem,” that detailed the challenges that even the most expert radiologist faces when trying to decipher a mammogram image.
“Looking at a mammogram is conceptually different from looking at images elsewhere in the body,” Memorial Sloan-Ketterer Cancer Centre radiologist Dr. David Dershaw told Gladwell in that piece. “Everything else has anatomy—anatomy that essentially looks the same from one person to the next. But we don’t have that kind of standardized information on the breast.
“The most difficult decision I think anybody needs to make when we’re confronted with a patient is: Is this person normal? And we have to decide that without a pattern that is reasonably stable from individual to individual, and sometimes even without a pattern that is the same from the left side to the right.”
The point of the article was that humans place too much trust in pictures as revealing “truth.” The picture that emerges from a mammogram is particularly open to interpretation.
Gladwell highlighted eye-opening findings from the University of Washington Harborview Medical Centre as to what happened when 10 radiologists were asked to interpret the same 150 mammograms.
One caught 85 per cent of cancers in the images right away. Another caught 37 per cent. Some saw many things to worry about, others saw none. In one case, three radiologists deemed a lump visible in the image to be normal, two others saw it as abnormal but probably benign, four weren’t sure, and one was certain it was cancerous.
Mammography does save lives. But not many, as it turns out. If 1,000 women who are age 60 right now have an annual (and let’s presume correctly interpreted) mammogram every year for the next decade - 10,000 mammograms, with all the expense that entails - breast cancer deaths among the group could be expected to drop from nine to six.
Nobody can blame us for wanting a fail-safe test that catches cancer early. Alas, the science isn’t there yet, and at any rate something new will likely be killing us by that point. Such is the nature of the human condition.
Preventing breast cancer remains important, of course. But so much of prevention comes down to personal responsibility - for what you eat; how often you exercise; how much you weigh; your alcohol consumption.
The world will rejoice when they come up with a screening program that corrects for bad habits. Until then, take care.