Increasing incidence of breast cancer: distinguishing between the effects of birth cohort and a national breast screening programme.

The incidence of breast cancer in post-menopausal women has been affected by the introduction of national breast screening programmes. The study describes the incidence of breast cancer in Scottish women aged 50-64 by year of birth before, during, and after the prevalent round of screening. Breast cancer registrations in Scotland for women aged 45-69 years from 1977 to 2003 were obtained. Birth cohort incidence rates were calculated and interpreted in the light of screening patterns at particular calendar time points. In the years before screening, there was a small rise in breast cancer incidence by birth cohort in women aged 50-54 which was not seen in other ages. During the prevalent screening round, incidence increased significantly with increasing birth cohort and thereafter continued rises in incidence by birth cohort occurred. The observed rise in breast cancer incidence among post-menopausal women is likely to be due to both screening effects and a true increase in incidence.