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75% of Canadians diagnosed with thyroid cancer don't have it, says new study

Overdiagnosis likely due to improvements in imaging technology and overtesting, say researchers

By Tricia Lo, CBC News

Researchers point to the rapid increase in thyroid cancer incidence especially since 1990, the variation among provinces and the peak in middle-aged women, saying these trends do not correspond to any known cause or risk factor for thyroid disease and instead likely stem from overdiagnosis. (Ryan Remiorz/Canadian Press)

Three out of four Canadians diagnosed with thyroid cancer don't actually have it and are being misdiagnosed because of the increased use of ultrasound imaging, says a new study published by University of Calgary researchers.

​Doctors Dawnelle Topstad and James Dickinson say frequent thyroid testing, combined with improvements in imaging technology, are leading doctors to mistakenly classify "clinically unimportant lesions" as cancer.

​"Doing extra testing finds extra things, even when they're not important things," Dickinson, a professor at the Cumming School of Medicine at the University of Calgary, said in an interview with CBC Calgary News at 6. "If doctors find small thyroid cysts, they may diagnose it as a possible cancer even though the cysts would never grow, invade or spread." Diagnosis Rates Jump, Death Rates Unchanged. The study — posted online by the Canadian Medical Association Journal on Monday — finds that thyroid cancer diagnosis rates have jumped in recent years, while death rates have remained stable, even though treatment protocols have remained unchanged.

This chart depicts the age-standardized incidence rates and crude mortality rates for thyroid cancer in Canada from 1970 to 2012, by sex. (CMAJ Open)

Between 1970 and 2012, thyroid cancer incidence jumped from 3.9 to 23.4 per 100,000 women, and from 1.5 to 7.2 per 100,000 men, according to the National Cancer Incidence Reporting System and the Canadian Cancer Registry. But mortality rates for both sexes have remained stable at around 0.5 per 100,000 for both sexes, cites the study. "If improved treatment for thyroid cancer is the reason for the mortality rates being constant, changes in treatment should coincide with the changing incidence. ​Despite debate over the past 40 years about the extent of surgery, the use of remnant radioactive iodine and suppressive thyroid hormone therapy, treatment protocols for thyroid cancer have not changed," write authors Topstad and Dickinson. With a much greater incidence of thyroid cancer in women, treatment would need to be much more effective in women than in men for similar mortality rates to be observed in both sexes, write the authors. But thyroid cancer treatment is identical for women and men. The authors therefore conclude it is unlikely that more effective treatment explains the stable mortality rates. 'Something Funny Going On' The authors estimate that at least 75 per cent of the population has been misdiagnosed. That figure is based on the assumption that actual thyroid cancer rates today should be consistent with those observed in 1990, before ultrasound tests became readily available in Canada.

This chart shows that middle-aged women in particular have noticed the largest increase in thyroid cancer diagnosis since 1972. (CMAJ Open)

"We know of no cause [for the observed increase in thyroid cancer diagnosis]. All of the standard causes that we know of for thyroid cancer haven't really changed," Dickinson said. Moreover, Dickinson said that, in countries that don't have readily available ultrasound testing, disease incidence rates remain the same. The researchers also note that incidence rates varied widely between provinces and even between regions within a province from 1992 to 2012.

They could not identify any geographic risk factors that could explain these differences. Instead, the authors conclude the differences are "likely due to varying rates of overdiagnosis arising from differences in practice."

Age-standardized incidence rates for thyroid cancer by province in 2012 show that Ontario had the highest incidence rates, more than double those in B.C. (CMAJ Open)

​"There's something funny going on here, and it's got much more to do with the behaviour of doctors and patients than it has to do with disease," Dickinson said. "It just doesn't make sense in terms of this distribution of disease, except in terms of how the healthcare system works or doesn't work." Costly Misdiagnosis Thyroid cancer treatment can include surgery and radioactive iodine (radioiodine) therapy, which have substantial side-effects and dangers. Post-surgery patients, for example, must take hormone treatments for life. The authors say further efforts to reduce these negative consequences should aim to reduce the unnecessary use of diagnostic imaging. They also say doctors must find better ways to distinguish between unimportant tumours and aggressive thyroid cancer that needs treatment. According to Dickinson, doctors should be wary of performing tests that could yield false positive results, and patients need to start asking professionals whether prescribed tests are truly necessary. Dickinson recommends that patients and doctors follow Canada's Choosing Wisely recommendations, which encourage conversations about unnecessary tests, treatments and procedures. "Don't do thyroid scans unless you're feeling something, a lump, there. Don't do 'em just because you've got a slightly abnormal thyroid test." ​Sources: CBC News

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