CALGARY -- at the University of Calgary suggests most people treated for thyroid cancer have tumours that would never pose any harm.
Dawnelle Topstad and James Dickinson, with the university's Cumming School of Medicine, pored over four decades worth of Canadian data on the disease.
They found thyroid cancer incidence rates in Canada increased by almost six times in women and five times in men between 1970 and 2012.
There was a big spike after the early 1990s, when ultrasound technology came into wide use. The sharpest increase was in women between 40 and 60.
At the same time, the rates of people who died from the disease were low throughout the period, with little variation between men and women.
"The likely cause of the increase in incidence is an overdiagnosis epidemic for clinically unimportant lesions detected by modern diagnostic imaging," Topstad and Dickinson wrote.
Dickinson said roughly 75 per cent of cases are harmless. Abnormal cells may be present, but the mass would never grow or would grow too slowly to cause any problems.
"The tendency for all of us is to say 'Well, we've got to make sure that we don't miss any cancers.' And that's entirely appropriate," he said in an interview. "But the difficulty is that then leads to over-calling. That leads to overdiagnosis and causes harm."
Medical professionals have had a hard time balancing the risks of the tumours themselves against the risk of treating them, he said.
"It's not easy, but we have to face up to it," he said. "At the moment, by not having been critical enough, we've now created a lot of harm."
Treatment for thyroid cancer can include surgery and radioactive iodine followed by lifelong hormone replacement drugs. Operating on the thyroid gland could endanger vital blood vessels and nerves in the area if there are complications.
"The neck is tiger country," Dickinson said.
He said the effects of radioactive iodine are fairly isolated and short-lived.
In the long term, it's a delicate balance to replace the hormones that the gland produced, since the wrong dosage can have dangerous side effects. For instance, an overabundance can lead to high blood pressure.
"You're then tied to the medical system for the rest of your life," said Dickinson.
The study noted sharp regional differences -- Ontario had a thyroid cancer incidence rate in 2012 twice that of British Columbia. After discounting possibilities such as increased radiation exposure, obesity and diabetes rates, Dickinson said the researchers suspect the availability of ultrasound testing was the cause.
As for why the rates are so much higher in women than men -- 23.4 per 100,000 in 2012 for women compared to 7.2 per 100,000 that year for men -- Dickinson said it could be that women are more likely to visit their doctors if they feel something's amiss.
"Further effort should concentrate on reducing overuse of diagnostic imaging and finding better ways to distinguish those patients with unimportant indolent tumours, while still identifying aggressive thyroid cancer that needs treatment," Topstad and Dickinson wrote in the paper, which was published this week in CMAJ Open.
Dickinson said there are lessons to be drawn for family physicians, radiologists, pathologists and surgeons.
"Everybody plays their part in focusing on treating the right ones and not treating the others."
