TRENDS IN BREAST-CANCER INCIDENCE IN SWEDEN 1958-1988 BY TIME PERIOD AND BIRTH COHORT

Statistics from the Swedish National Cancer Registry based on all 110, 658 cases of invasive breast cancer during the 31-year period 1958 198 8 were analysed. Age-specific incidence rates increased over successiv e calendar periods. The average annual increase in the age-standardise d incidence rate was 1.3%, with the greatest percentage changes among the youngest age groups. During the latter half of the study period, t he rates of increase tended to diminish in the youngest age groups and even reversed significantly among women from 75 years of age. In anal yses using age-period-cohort models, the best fit of the cancer incide nce data was found for the full model which simultaneously considered the effects of age, period and cohort. Cohort effects were found to be more important than period effects, in terms of model fit. These effe cts emerged as a seemingly consistent, and in a logarithmic scale, fai rly linear increase in the relative risk of breast cancer incidence wi th a 3-fold elevation in women born in the 1950's relative to those bo rn in the 1880's It is concluded that the rising breast cancer inciden ce in Sweden is explained chiefly by birth cohort effects, which indic ate persistent secular changes in largely unknown risk factors associa ted with life style. We could not in the present data see any clear ev idence for an adverse effect of contraceptive or replacement sex stero ids on breast cancer incidence.