Colon cancer trends in Norway and Denmark by socio-economic group: A cohort study

Aims: Norway has experienced an unprecedented rapid and so far unexplained increase in colon cancer incidence. Norwegian rates passed Danish rates for men in 1985 and for women in 1990. This study aimed to unravel clues to the development in colon cancer incidence by investigating changes over time in incidence by socio-economic group. Methods: Persons participating in the 1970 censuses in Norway and Denmark were aged 55–75 years in 1971–1980 (called pre-crossing period) and in 1991–2000 (called post-crossing period), respectively. Country, sex, age and socio-economic group-specific colon cancer incidence rates. Percent change in the average rate from the pre- to the post-crossing period. Results: In the pre-crossing period, Norwegian male managers/administrators had the highest colon cancer incidence, but the largest increase in incidence from the pre-to the post-crossing period was seen for unskilled workers, skilled workers and farmers. The rate for unskilled workers almost doubled and in the post-crossing period this rate had bypassed that of managers/administrators. A similar development was seen for Norwegian women and was less dramatic in Denmark. Conclusions: The change in the risk of manual workers has been the driving force behind the dramatic increase in the Norwegian incidence of colon cancer. This development resulted in a reversal of the socio-economic gradient from the classic European pattern with the highest incidence in the upper socio-economic groups to an American pattern with the highest incidence in the lower socio-economic groups. This ‘Americanization’ of the disease pattern followed the rapid growth in the Norwegian gross domestic product.