IT'S JUST COSMETICS:

Turtles are part of what H. Gilbert Welch, a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice, calls the barnyard pen of cancers. The barnyard has three animals, turtles, birds, and rabbits. "The goal of early detection is to fence them in," he says.

You can't fence in the birds. They're the super aggressive lethal cancers that are beyond cure. The rabbits, you can maybe do something about if you can spot them and treat them. (Treatments that, by the way says Welch, have gotten better and better.) "But for the turtles," he says, "you don't need fences because they're not going anywhere anyway. And the thyroid is full of turtles."

The breast and the prostate are full of turtles too, and just as the thyroid-scanning ultrasound devices are more likely to find little nodules there, an upsurge in mammography has led to a corresponding upsurge in something called ductal carcinoma in situ. Basically cancers that most of the time would just sit around and do nothing if you left well enough alone. In other words, they're indolent (great word), not malignant. Whether or not we treat them (or even look for them) has been a matter of great debate in recent years.

It's very hard to know when upticks and outbreaks are quite what they appear. Even infectious disease outbreaks can sometimes be attributed to more-sensitive screening methods. The rise in whooping cough cases has multiple causes of course, but one of them is improved screening methods. Gene-based tests called PCR assays can inflate the number of actual diagnoses, according to a piece by epidemiologist James Cherry in the New England Journal of Medicine.

Which is to say, if you start looking for something carefully, and if you use better methods to see what you're looking for, you will often find it.

Welch points to an example from the '70s, when some employees at Lawrence Livermore National Labs (who deal with nuclear stuff all day long) were diagnosed with melanoma. Cancer cluster! But no--it was something else.

What happened, Welch writes in his book Should I Be Tested for Cancer, is that one person probably got sick. Then, other people in the lab started getting checked for moles. Some were funky, so that leads to biopsies, which leads to, in some cases, an actual diagnosis. Then people start really getting worried. The lab kicks off an awareness campaign, so more people go in for checks, leading to more biopsies. "The whole epidemic looked subsequently like it was a pseudoepidemic," he says. "It was an epidemic of diagnosis." The melanomas were mostly turtles.

In South Korea, checking more thoroughly has absolutely led to more diagnoses of thyroid cancer. In the late '90s, doctors in South Korea started screening people for thyroid cancer (it was an add-on test to the national cancer screening program), and cancer cases took off. "There was a 15-fold increase," says Welch. "There was nothing like it in the world!" Now, he says, thyroid cancer is the most common cancer in Korea--more common than breast, and colon, and lung.

Here's the really pernicious part. People get checked for thyroid cancer, doctor finds a little nodule, does a biopsy, there's some cancery stuff in there, so they remove the thyroid, and the person--saints be praised!--the person lives. (Because of course they lived, they just had a little thing that would never have been a problem in the first place.) They live (without their thyroid) and now they are a survivor and the survival rates for thyroid cancer in South Korea are now really high. Great, right? No. "Once you understand the problem of turtles, you understand you're giving credit to finding the cancers that don't matter," says Welch.

How did South Korea combat this surge in cancer cases? A group of doctors (including Welch) wrote a letter in 2014 discouraging screening with ultrasonography. Poof. Thyroid operations dropped by 35 percent in a year. Because the best test "isn't one that finds the most cancer," he says. "The best test is one that finds the cancers that matter."

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