Risk of thyroid cancer among Iranian immigrants in Sweden

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.

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.

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.

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.