Thyroid cancer incidence in Canada: a national cancer registry analysis

Thyroid cancer incidence rates are increasing in many developed countries while mortality rates remain stable. International evidence shows that the increase in incidence rates is mostly caused by overdiagnosis of small papillary cancers. We sought to describe how thyroid cancer incidence has changed and how it varies between provinces in Canada.

Data were obtained from the National Cancer Incidence Reporting System, causes of death tables and the Canadian Cancer Registry using the 1991 census population structure. We report thyroid cancer incidence by sex, age and province and mortality by sex from 1970 to 2012.

Since 1970, age-standardized thyroid cancer incidence rates have increased in women from 3.9 to 23.4 per 100 000 and in men from 1.5 to 7.2 per 100 000 while mortality rates have remained stable at around 0.5 per 100 000 for both sexes. In 2012, incidence rates for both women and men were highest in Ontario (31.5 and 9.2 per 100 000, respectively) and lowest in British Columbia (13.2 and 4.5 per 100 000, respectively). Age-specific incidence rates were the highest in Ontarian women aged 50-54 years, at 65.2 per 100 000.

The rapid increase in thyroid cancer incidence especially since 1990, the variation among provinces and the peak in middle-aged women does not correspond to any known cause or risk factor for disease, although the lack of change in mortality rates suggests that serious thyroid cancer has not increased. The likely cause of the increase in incidence is an overdiagnosis epidemic for clinically unimportant lesions detected by modern diagnostic imaging. To reduce the harms of overtreatment, overdiagnosis should be reduced, through more judicious use of diagnostic imaging.