Interpreting the international trends in testicular seminoma and nonseminoma incidence

There are considerable geographic, ethnic and temporal variations in the global incidence of testicular cancer. The disease mainly affects Western populations, with average rates in developed areas of the world six times higher than those in developing areas. About 500,000 new cases were diagnosed worldwide in 2002, with the vast majority being germ cell tumors and occurring in young adult males. Traditionally, these tumors are further classified into seminoma and nonseminoma. In this Review, trends in the incidence of germ cell tumors are examined using high-quality cancer-registry data from 41 populations within 14 countries worldwide. To assess whether trends of seminoma and nonseminoma incidence are similar, data were analyzed by birth cohort. These analyses should reveal similar trends if the 10-year difference in the clinical manifestation of cancer between subtypes is caused by differences in the speed of progression from the same early rate-limiting step to the onset of symptomatic disease. In each country, incidence has uniformly increased in successive generations born from around 1920 until very recently. Cohort-specific trends in seminoma incidence are similar to cohort-specific trends in nonseminoma incidence, lending support to the conclusion that the subtypes are epidemiologically and etiologically comparable. The findings presented are related to current theories and evidence regarding the determinants of testicular germ cell cancer.