Age-period-cohort modelling of breast cancer incidence in the Nordic countries

The Nordic countries have experienced a steady increase in breast cancer in cidence throughout the past 35 years. We analysed the incidence in Denmark, Finland, Norway and Sweden during the period 1958 to 1992 using age-period -cohort models and taking the systematic mammography screening into account . Assuming the age dependency of the incidence pattern in old age to be com mon for the Nordic countries, an internal comparison could be made among th e four countries of the cohort effects and the period effects. The study in dicated that the period effects have been of importance for the increase in breast cancer incidence seen in the Nordic countries. The widespread pract ice of neglecting the period effects in age-period-cohort analysis of time trends in breast cancer incidence therefore probably needs reconsideration. A key finding was that Danish women born in the 20th century seem to have been exposed to an increasing load of cohort borne breast cancer risk facto rs not experienced to the same extent by Norwegian women, whereas they were seemingly subjected to the same period effects. Copyright (C) 2001 John Wi ley & Sons, Ltd.