Ekbom, A, L Richiardi, O Akre, SM Montgomery and P Sparén. 2003. Age at Immigration and Duration of Stay in Relation to Risk for Testicular Cancer Among Finnish Immigrants in Sweden. Journal of the National Cancer Institute 95:1238–40.

Testicular cancer is increasingly common in a number of countries, including in North America and Europe. Some countries have markedly higher rates. For example, testicular cancer is more than twice as high in Sweden as it is in Finland, and twice as high in Denmark as it is in Sweden.

With this new study, examining the incidence of testicular cancer in men who have immigrated from Finnish to Sweden, Ekbom and his colleagues provide strong evidence that environmental exposures early in life are likely to be major determinants for this disease. Their findings are consistent with Skakkebæk's theory that the origins of testicular cancer and several other testicular maladies are part of single syndrome, testicular dysgenesis syndrome, whose origins lie in disruption of the development of the fetal testis.

What did they do? Ekbom et al. compared the incidence of testicular cancer in Finnish immigrants to Sweden with the incidence rates of the general Swedish population. In particular, they were interested in whether men moving to Sweden became at risk to testicular cancer at rates comparable to Swedish men, or whether they more closely resembled men in their native Finland.

They obtained data from the national health registry of Swedish residents, which includes information not only about health status but also, for immigrants, about year and origin of immigration. Using these data, they determined the incidence of testicular cancer in the immigrants, and compared them to data from the general Swedish population.

What did they find? The cohort studied included 93,172 men who had immigrated from Finland to Sweden from 1969 to 1994. Finnish immigrants in Sweden had fewer than half the number that would have been expected based on testicular cancer rates in Sweden. The ratio of observed cases to expected cases was 0.34 (95% confidence interval 0.21-0.53). The incidence in the immigrants did not differ from the incidence rates in Finnish men who had not emigrated to Sweden. Neither age at immigration nor duration of stay affected the results.

What does it mean? The authors interpret their results to implicate a combination of genetic and environmental factors. Genetic differences may contribute to the overall difference between Finland and Sweden, but they cannot explain the increasing incidence of testicular cancer, nor differences between Finnish and Estonian men or Swedish and Danish men (who are more similar to one another genetically than Finnish men are to Swedish men).

The fact that neither age at immigration nor duration of stay altered the results strongly implicates events very early in life. The authors observe: "Thus, it is likely that differences between the populations with respect to exposure of environmental factors early in life explain the low incidence of testicular cancer among Finnish immigrants. It is highly plausible that susceptibility to these exposures occurs in utero when the testicles are developing and differentiating in the presence of numerous growth-regulating factors."

While Ekbom et al.'s results strongly implicate environmental exposures early in life, they do not identify what exposures might be responsible. They comment that because the risk of testicular cancer is largely determined in utero, the differences observed today between incidence rates in Sweden vs. Finland "would be attributable to environmental differences that were present approximately 30 years ago."

These results are consistent with a recent study finding that testicular cancer risk is more tightly associated with organochlorine levels in a mother's serum than a son's, both measured at the time of diagnosis of testicular cancer in the son.