Highlights
- There is substantial geographic variation in female thyroid cancer incidence across Ontario, with the highest rates in and around the Greater Toronto Area.
- Thyroid cancer incidence has been increasing in Ontario over the past two decades.
- Increases may be partially due to better diagnostic technology, which may allow for the detection of sub-clinical tumours.
A recent analysis found that thyroid cancer incidence rates in females varied greatly among census divisions (CDs) in Ontario from 1999 to 2013. Overall, the CDs surrounding and including Toronto (Peel, Halton, Toronto and York) had the highest rates during this time period, and rapidly increasing rates were observed in northeastern and southwestern Ontario, especially in Algoma, Cochrane, Chatham-Kent and Essex.
In 1999–2003, the age-standardized incidence rate (ASIR) for females ranged across CDs from 7.4 per 100,000 in the combined Lennox and Addington, Frontenac and Renfrew to 32.1 per 100,000 in York. For the 5 most recent years of the analysis, 2009 to 2013, ASIRs ranged from a high of 68.1 per 100,000 in Algoma to a low of 13.2 per 100,00 in the combined Prescott and Russell, Stormont, Dundas and Glengarry CDs.
Similar to the pattern observed for all-stage thyroid cancer incidence, the CDs surrounding and including Toronto (Peel, Durham, Halton, York and Toronto) had the highest ASIR for early-stage thyroid cancer. Thyroid cancer was more likely to be diagnosed at stage 1 with approximately three-quarters of cases in 2013 diagnosed at this early stage. Only 5% of thyroid cancer cases were diagnosed at stage 4.[1]
There was a rapid increase in thyroid cancer incidence throughout the province, between 1999–2003 and 2009–2013, with increases in incidence observed in every CD in Ontario. The rising incidence rate of thyroid cancer has been linked to improved diagnostic technology, including the use of ultrasound and fine-needle aspiration, which may have allowed for the detection of subclinical tumours.[2][3][4][5] This association has been observed in Ontario, where there are large differences across the province in the use of discretionary medical tests, including diagnostic ultrasound,[6] and the CDs corresponding to the regions with the highest test use were also the CDs with the highest incidence.[5]
Unlike most common cancers, thyroid cancer incidence is higher in females than in males. A number of possible reasons for this higher incidence in females have been proposed in the literature. Males and females have biological differences in their hormone levels, including thyroid stimulated hormone and sex steroids,[7][8][9] so females are more likely to have thyroid disease due to higher hormone levels. Additionally, females seek medical attention more often than males and participate more actively in medical visits,[10][11][12] which means they are more likely to get diagnosed.[13]
Healthcare providers and policy-makers must consider the treatment implications and possible long-terms effects of the rising burden of thyroid cancer as they engage in planning for the healthcare system and patients, especially for people diagnosed at an early stage.