Thyroid cancer incidence among Asian immigrants to Ontario, Canada: a population-based cohort study

Background — The highest rates of thyroid cancer are observed in Pacific Island nations, as well as Australia and Asian countries bordering the Pacific. We sought to determine the risk for thyroid cancer for immigrants to Canada from Southeast and East Asia, compared to immigrants from other regions and nonimmigrants.

Methods — We conducted a population-based longitudinal cohort study using healthcare administrative data, examining all residents of Ontario without pre-existing thyroid cancer. Individuals were followed from January 1997 or five years after they became eligible for healthcare coverage in Ontario, whichever came later. Patients were followed until March 2015 for incident differentiated thyroid cancer, and then for recurrence.

Results — The study followed 14,659,733 individuals for a median of 17.0 years. Thyroid cancer incidence was 43.8 cases per 100,000 person–years among Southeast Asian immigrants, 28.6 among East Asian immigrants, 21.5 among other immigrants and 14.5 among nonimmigrants. Incidence was highest among immigrants from the Philippines (52.7), South Korea (33.5) and China (30.0). Adjusted hazard ratios for thyroid cancer compared to nonimmigrants were 2.66 (95% CI 2.48–2.84) for Southeast Asian immigrants, 1.87 (1.75–2.00) for East Asian immigrants and 1.51 (1.45–1.57) for other immigrants. Immigrants were more likely to have papillary histology and stage I cancer. East Asian but not Southeast Asian immigrants had a lower risk of recurrence (hazard ratios 0.73 [0.57–0.94] and 1.01 [0.81–1.26], respectively).

Conclusions — Immigrants from Southeast and East Asia had markedly higher thyroid cancer incidence than nonimmigrants. At particularly elevated risk were immigrants from the Philippines, South Korea and China.