Thyroid cancer may be one of the fastest growing cancers in Canada, but a new report suggests that up to 90 per cent of these cancer cases are “overdiagnosed” and shouldn’t be treated at all.

The report by the International Agency for Research on Cancer, or IARC, suggests that the so-called growing epidemic of thyroid cancer in high-income countries is likely simply due to overdiagnosis.

The report, published in the New England Journal of Medicine, says “indolent, non-lethal diseases” exist “in abundance” in the thyroid gland of healthy people.

But most of these tumours detected through sophisticated technologies such as MRI and CT scans are likely not harmful and "unlikely to cause death during a person's lifetime."

In total, the IARC researchers estimate that more than 470,000 women and 90,000 men may have been overdiagnosed with thyroid cancer during last two decades in the 12 countries they studied.

“People who are treated and people who are not treated but monitored, they had the same prognosis,” says report co-author and IARC staff scientist Salvatore Vaccarella.

The authors say overdiagnosis is a problem because many of these patients often have their thyroids removed (thyroidectomy) or undergo unnecessary lymph node dissection and radiation therapy.

Instead of being treated, many with low-risk tumours would be better off with “watchful waiting” -- meaning their tumours are left alone and simply monitored. It’s the same approach already promoted for early prostate and breast cancer

In 2015, an estimated 6,300 Canadians were diagnosed with thyroid cancer. The vast majority survived. In fact, the five-year relative survival for thyroid cancer in Canada is 98 per cent – meaning that people with thyroid cancer are 98 per cent as likely to live five years after diagnosis as the general population.

Theresa Marie Underhill was diagnosed in 1999 and now helps others through Thyroid Cancer Canada. She says “watchful waiting” is easier said than done when you’ve been told you have cancer.

“‘Let’s just wait and see what happens’ -- that is challenging. It’s challenging for many individuals and it's challenging for the health system,” she says.

Still, IARC Director Dr Christopher Wild says the drastic increase in overdiagnosis and overtreatment of thyroid cancer “is already a serious public health concern” and called for more research to decide on the best approach to thyroid cancer and to avoid unnecessary harm to patients.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip