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A new study shows why cancer screening can be bad for your health

Nurse preparing a patient for a CT scan.
Nurse preparing a patient for a CT scan.
Tyler Olson/Shutterstock

In 1999, South Korea launched an ambitious campaign to improve health by finding and treating diseases like cancer. Part of this effort included a state-subsidized, mass screening program.

In two decades, the number of South Koreans diagnosed with thyroid cancer rose fifteenfold.

At first glance, this seemed like a great success. But while South Korea managed to diagnose many more patients with the disease, the country didn't actually succeed at its mission: improving health.

Thyroid-cancer deaths remained stubbornly stable as the incidence of the disease skyrocketed. The additional screenings were leading to many more cases, but those additional diagnoses — and the invasive courses of treatment that came along with them — weren't making thyroid cancer any less deadly.

Cancer screening may actually do more harm than good

How do you get a message across that something you always thought was good for you — say, broccoli or exercise — is actually really bad for you, and could potentially make you very sick?

This is the question Dr. Gilbert Welch, a physician and researcher at Dartmouth's school of medicine, has been asking himself for years about cancer screening.

Over a decade ago, he began looking into the effects of mass screening programs that have emerged around the globe. These take otherwise healthy people and subject them to tests to find out whether they have lumps and bumps that may be malignant.

He found something disturbing: in many cases, screening wasn't actually helping people or saving lives.

His latest study of thyroid-cancer screening in South Korea, just published with Korean colleagues in the New England Journal of Medicine, adds yet more fodder for his crusade to make healthy people think twice about getting screened.

It shows what appears to be an epidemic of overdiagnosis in South Korea: lots more people were told they have thyroid cancer, but no more lives were saved. If screening programs worked as intended, more diagnoses would correlate with fewer deaths.

The chart below illustrates how that's not happening. As you can see, it looks as if there’s a surge in thyroid cancer in South Korea, yet the related death rate has held constant. "There is nothing like it in the world," Welch said of the astronomical rise.


(Chart courtesy of the New England Journal of Medicine)

In the US, during the same period, diagnoses of thyroid cancer more than doubled, but we also had a death rate that remained stable.

In South Korea, the reason for the surge in diagnoses is simple and it has nothing to do with biology or the environment: over the last twenty years, doctors there started looking more closely for thyroid cancer, a side-effect of the national screening program launched in 1999.

An "epidemic of diagnoses" in South Korea

Though thyroid ultrasounds weren't included in the state-funded screening campaign, doctors began promoting them as an inexpensive additional test while patients were getting screened anyway.

Right around that time, South Korean researchers started to note the "an epidemic of diagnosis," as Welch calls it, where the rate of thyroid cancer grew faster than any cancer, anywhere in the world.

They were seeing more and more cases, more and more cancer patients. These folks were getting invasive surgeries. "Any time you have a massive increase in diagnosis, you tend to have a massive increase in treatment, and all of our treatments have some harms," explained Welch.

Some were having their thyroids removed, and were being put on lifelong hormone medications. "Some had vocal chord paralysis from the operations," said Welch. "There is also death associated with the operation."

But again, all this treatment wasn't actually saving lives. "So not only is it scary for people, and does it label people as having a feared disease, but they end up getting unneeded treatments."

This finding shouldn't really be news. Since 1947, the medical community has had evidence that, while common, thyroid cancer is rarely a cause of death and that at least one third of adults can have small thyroid cancers that are harmless.

"Overdiagnosis" in thyroid cancer — and many others

Welch and his colleagues use the term "over-diagnosis" to refer to the diagnoses of cancers that would not have been fatal or even harmful. These are diagnoses that turned healthy people into cancer patients unnecessarily, leading them to unnecessary treatment and hospitalization, creating clubs of "cancer survivors" who actually would have lived even if their cancers were left in them.

Mass screening of healthy people is notably different from using technologies like ultrasounds to diagnose people at risk of a disease or who have symptoms that require investigation.

And not all mass screening is bad. Colorectal-cancer screening, for example, has been shown to save lives as a result of early detection, and the Pap test transformed cervical cancer into a treatable disease.

Still, Welch has found evidence of overdiagnosis in breast and kidney cancers, as well as melanoma. Population-based screening for prostate cancer has been scaled back after the realization that most men will die with, not of, the disease.

Welch even found overdiagnosis in lung cancer. "That's the wildest one," he said. "We all think of lung cancer as being the most aggressive and of course it can be."

But overdiagnosed lung cancer has been documented since the early 1980s. "The most powerful example of population-based screening was in Japan, where they started using spiral CT scanning to do mass screening and found the same rate of cancer in smokers as non-smokers."

For the past 50 years, we have known that smokers are at a 20- to 30-times greater risk of dying from lung cancer. "When you use a very powerful technology to screen for lung cancer, it looks like smokers and non-smokers are about the same."

How to avoid being a victim of overdiagnosis

In the US, Welch said harmless thyroid cancers are also being overdiagnosed often in the process of looking for something else. This usually happens when someone gets a chest CT scan or an ultrasound of the neck to evaluate their carotid artery (the blood vessel that supplies the brain). "These exams are meant to find things that might be relevant to stroke, for example, but the thyroid is right there and a lot of surprise cancers are found that way."

The South Korean experience, Welch said, should be a cautionary tale for the rest of the world, since many countries — including Canada, the US, Australia, China, France and Italy — have recorded a jump in thyroid-cancer incidence and no increase in related mortality in recent years.

"The South Korean experience suggests that these countries are seeing just the tip of the thyroid-cancer iceberg — and that if they want to prevent their own 'epidemic,' they will need to discourage early thyroid-cancer detection," Welch and his colleagues wrote in their study.

To avoid being the victim of overdiagnosis, Welch suggests people talk to their doctors about their individual risks before getting screened. The US Preventive Services Task Force, which examines the evidence and makes recommendations about mass cancer-screening programs, is also an excellent resource. (When the Task Force looked at the evidence, it recommended against mass screening of asymptomatic people for thyroid cancer since there was a "lack of evidence" that early detection improves outcomes.)

Getting the message across that not all cancer is deadly, and finding it early can actually do more harm than good, hasn't been easy, Welch said.

"All the forces line up to support early cancer detection," he explained. "On first inspection it makes total sense, it's so intuitively appealing. And then the more people who are diagnosed with thyroid cancer, who have their thyroids taken out, and who are doing well — other than the fact that they're on medications or may have vocal-chord paralysis — the more they are seen as survivors."

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