Temporal trends in breast cancer incidence and mortality in Australia: an age-period-cohort analysis

Background: Breast cancer is the most diagnosed cancer among Australian women. Recently, incidence rates have risen but mortality rates have decreased. The extent to which these changes are the result of cumulative risk factor effects (age-effects), events affecting all women at specific points in time (period effects), or changes in generational risk factors (cohort effects) is unclear. This study investigates whether observed trends in breast cancer incidence and mortality are associated with age, period, or cohort effects. Methods: Annual Australian breast cancer incidence (1982–2020) and mortality data (1907–2022) were obtained from the Australian Institute of Health and Welfare. Age-Period-Cohort (APC) modelling with a drift in cohort function was used to estimate adjusted age, period and cohort effects. Results: Age effects showed higher incidence and mortality rates with increasing age, peaking at 75–84 years. Cohort effects showed progressively increasing risk among women born after the 1940s, with higher incidence among younger cohorts. Incidence peaks corresponded with introduction of population-based mammography screening, and also changes in population level risk factors. Period effects (adjusting for cohort effects) were modest, demonstrating reductions in incidence over time, while mortality peaked in the 1990s before declining after 2000. Conclusion: Increasing age-related breast cancer incidence and mortality reinforce the importance of early prevention. Incidence has shifted due to cohort and period effects with younger generations showing the highest increases in incidence, suggesting generational shifts in breast cancer risk, likely attributable to mammography screening and increased prevalence of modifiable risk factors.