Trends in invasive breast cancer incidence among French women not exposed to organized mammography screening: an age-period-cohort analysis.

Background: The long tenure of the Doubs cancer registry (France) and the late implementation of a mass screening program provide a unique opportunity to assess the relative contributions of age, period and cohort effects to the increase in female invasive breast cancer incidence, while avoiding the influence of an organized screening program.

Methods: Population and incidence data were provided for the Doubs region during the 1978-2003 period. Breast cancer counts and person-years were tabulated into 1-year classes by age and time period. Age-period-cohort models with parametric smooth functions were fitted to the data, assuming a Poisson distribution for the number of observed cases.

Results: A total of 5688 incident cases of invasive breast cancer in women were diagnosed in women aged 30-84 years in the Doubs region between 1978 and 2003. The annual percentage increase in incidence is 2.09%. Age effects rise dramatically until age 50, and at a slower pace afterwards. Large cohort curvature effects (p<10(-6)), show departure from linear trends, with a significant peak for women born around 1940. Period curvature effects are lower in magnitude (p=0.01).

Conclusion: Both cohort and period effects are involved in the marked increase in breast cancer incidence over a 25-year period in the Doubs region. Although the future trend for breast cancer incidence is difficult to predict, the introduction of an organized screening program, and the sharp decline in hormone replacement therapy use will likely contribute to period effects in future analyses.