Risk of thyroid cancer among Iranian immigrants in Sweden.

Objectives: Studies of immigrants are of value to elucidate the role of environmental factors in cancer causation, but large cohorts are needed in order to study rare cancers. We conducted a register-based study of a cohort of 59,274 (32,236 men and 27,038 women) Swedish residents born in Iran, with follow-up between 1969 and 2004. We identified 50 incident cases of thyroid cancer during more than 800 thousands person-years of observation.

Methods: Rate ratios (RR) were calculated based on Poisson models estimated by the maximum likelihood method, using Swedish born residents with both parents born in Sweden as reference population. 95% confidence interval was estimated on the assumption that the observed numbers of cancers follow a Poisson distribution.

Results: The adjusted RR of thyroid cancer among Iranian immigrants was 2.6 (95% CI 2.0-3.5), without appreciable sex differences. In both sexes, the excess risk was highest among people who were younger than 30 years at immigration. Among women, the largest excess risk (adjusted RR = 4.6, 95% CI 2.9-7.4) was observed during the first 5 years from immigration, while for men, during the second decade from immigration. The rate ratio was higher among subjects who immigrated before 1990 (adjusted RR = 2.7, 95% CI 2.0-3.8) than among those immigrated thereafter, particularly among men.

Conclusion: The observed excess risk among Iranian immigrants compared to Swedish-born residents is compatible with differential burden of environmental risk factors, the most likely of which are iodine deficiency and high natural levels of ionizing radiation.