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Evan Horowitz

Why don’t screening tests seem to help against cancer?

Cancer sucks. Best thing is not to get it, and the second best thing is to detect it quickly. As the American Cancer Society puts it, “Finding cancer at its earliest, most treatable stage gives patients the greatest chance of survival.”

Trouble is, this isn’t always true. Sometimes, an early screening does more harm than good, picking up slow-growing tumors and encouraging aggressive treatment of cancers that would never have proved life-threatening. Surprisingly few screening tests have been shown to actually help people live longer, healthier lives.

Why doesn’t screening work?

Whether it’s a mammogram, a pap test, or a skin cancer evaluation, the point of a screening test is to spot cancer early. And sometimes, that approach does work. Colonoscopies, for instance, seem to be helping people avoid colon cancer.


However, in many other cases screening just doesn’t seem to have that big an impact. Last month, a team of researchers from Stanford published a review of screening tests — for a variety of different diseases, cancer among them — and they found little evidence that screening actually helps patients live longer.

If early detection really is the key to effective treatment, then these results may seem puzzling. But there are a number of reasons why screening tests don’t always lead to better outcomes.

- Finding harmless cancers. Sometimes, screening tests pick up slow-growing cancers that pose no real threat. One example is the blood test that was used to check for prostate cancer. Not only was it picking up a lot of cancers that were never going to harm patients, it led to treatment that was causing some men to lose their bowel control and sexual function.

- False positives. When you’re looking for relatively rare conditions, like particular cancers, even very accurate tests can produce lots of false positives. That, in turn, spurs further tests and treatments, some of which, like CT-scans, may actually increase cancer risks.


- Testing too young. While cancer is quite prevalant among older people, the odds of getting it before your 50th birthday are only around 5 percent. So when you screen younger people for cancer, you greatly increase the likelihood of false positives. This has proved particularly controversial in the case of mammography, after a government-sponsored panel of experts suggested women shouldn’t start getting routine mammograms until age 50 (previsouly, it had been 40).

- Limited treatment options. If a screening test does pick up cancer, but there’s no good treatment, then it really doesn’t help.

- Humans are mortal. Sometimes, finding and treating cancer just increases the likelihood that the patient will die of something else. The fact that cancer overwhelmingly affects people in their golden years makes this even more likely.

Does this mean we’re losing the fight against cancer?

No. Recent decades have seen a big drop in cancer deaths.

Still, screening techniques and new treatment protocols can claim only part of the credit for that decline. Probably the biggest reason cancer deaths are receding is the success of antismoking campaigns, which have dramatically reduced lung cancer rates (particularly among men.)

So should we stop screening for cancer?

In some cases, the answer is yes. The use of prostate cancer screening tests has already started declining, for example.

What’s really needed is better evidence. Screening tests could still make a dramatic difference in the fight against cancer, but we need to know which tests are effective. That means something more than a test which can reliably spot cancer cells. It means a test that has been proven to actually give people more quality years. And for now, few tests meet that standard.


Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the United States. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz