News

The incidence of thyroid cancer is rising more quickly than any other cancer in Canada.

But new research suggests that the whopping increase - 150 per cent in the past decade - is due almost solely to the incidental detection of tumours during other tests, and that most of those growths are harmless.

"There is no epidemic of thyroid cancer," said Stephen Hall of the department of otolaryngology of the Cancer Research Institute at Queen's University in Kingston, Ont.

"The epidemic is in the detection of smaller asymptomatic cancers," he said.

The result may be that a lot of people are being treated unnecessarily, Dr. Hall said.

The study, published in today's edition of the Canadian Medical Association Journal, involved the examination of 7,422 cases of thyroid carcinoma in Ontario between 1990 and 2001. Researchers examined pathology reports of 10 per cent of the patients, chosen at random.

The research showed that the annual number of cases of thyroid cancer increased to 990 from 403 over the 12-year period. But virtually all that increase was in tumours measuring two centimetres or less.

By contrast, the number of tumours measuring two to four centimetres remained stable.

Dr. Hall said virtually all the small tumours were detected during other medical imaging tests. A typical scenario involved the victim of a motor vehicle collision getting a CT scan for a neck injury and a small tumour showing up on the thyroid.

"These are incidental findings that result in a whole cascade of interventions," he said.

Rita Banach, founder of Thry'vors, the Canadian Thyroid Cancer Support Group, said she was unconvinced by the argument.

"Just because a lot of the tumours being found are small doesn't mean they are harmless and don't need treatment," she said.

Ms. Banach said there is only one known cause of thyroid cancer - acute exposure to radiation. "We're not living in Chernobyl, so something else is going on here."

But there are many theories of other causes, such as exposure to low-level doses of radiation in the environment (including the practice of treating many childhood illnesses with radiation prior to 1960), iodine deficiency, genetics and a personal history of goiter or thyroid nodule.

Ms. Banach also noted that women are about five times more likely than men to be diagnosed with thyroid cancer, which has led to speculation that there is an underlying hormonal cause.

The new research suggests that women are diagnosed in much larger numbers because they undergo far more diagnostic tests.

In a commentary also published in today's edition of the CMAJ, Jacques How of the division of endocrinology at Montreal General Hospital said there is no doubt that much of the rise in thyroid cancer is due to changes in medical practice such as more frequent imaging.

But he said that is no reason to be complacent and assume that all small tumours are benign. "We need to find a compromise between overdiagnosis and beneficial early screening."

He also said that even if the increase in thyroid cancer is real, and not an artifice of diagnostic testing, the good news is that thyroid cancer is highly treatable and curable.