Sociodemographic characteristics associated with thyroid cancer risk in Canada.

BACKGROUND: Thyroid cancer incidence in Canada has increased rapidly over the past 25 years. This study examines thyroid cancer incidence and relative risk according to individual-level sociodemographic characteristics in two population-based cohorts. DATA AND METHODS: The analysis uses data from the 1991 and 2001 Canadian Census Health and Environment Cohorts (CanCHECs). Using nine years of cancer follow-up for both time periods, age-standardized incidence rates of thyroid cancer were estimated by sex-with sex-specific estimates produced by immigrant status, ethnicity, educational attainment and family income-and by histology type. All characteristics were included in sex-specific standard Cox proportional hazard models to examine the relative risk of thyroid cancer and the relative risk of papillary versus non-papillary thyroid cancer. RESULTS: A significant increase over time in thyroid cancer incidence was observed for both sexes, and across all characteristics. Immigrant status and ethnicity were each independently associated with the risk of thyroid cancer, with immigrant men and women and East and Southeast Asian women at higher risk. Men and women who had a postsecondary diploma or higher or who were in the highest income quintile were at increased risk of being diagnosed with papillary thyroid cancer, but not with non-papillary thyroid cancer. DISCUSSION: While increased detection has played a role in the rising incidence of thyroid cancer in Canada, it does not fully account for the greater relative risk among the immigrant population and certain ethnic groups. More research is needed to better understand the determinants of the increased risk in these populations.