The increase in thyroid cancer diagnosis in Canada despite stable mortality rates may be due to overdiagnosis following diagnostic imaging, according to a study published in CMAJ Open.
Researchers from Canada sought to describe how thyroid cancer incidence has changed and its variances between provinces in Canada, using data obtained from the National Cancer Incidence Reporting System, causes of death tables and the Canadian Cancer Registry using the 1991 census population structure.
The results showed an increase in age-standardized thyroid cancer incidence rate among women from 3.9 per 100,000 in 1970 to 23.4 per 100,000 in 2012. Among men, the incidence rose only from 1.5 per 100,000 to 7.2 per 100,000 in that same period. Mortality rates have remained stable at around 0.5 per 100,000 for both sexes from 1970 to 2012. 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 to 54, at 65.2 per 100,000.
The researchers concluded that the rapid rise of thyroid cancer diagnosis among women does not correspond to any known cause or risk factor for disease. 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 over diagnosis epidemic for clinically unimportant lesions detected by modern diagnostic imaging,” they wrote. “To reduce the harms of overtreatment, over diagnosis should be reduced, through more judicious use of diagnostic imaging.”