Breast cancer incidence trends in Golestan, Iran: An age-period-cohort analysis by ethnic region, 2004-2018

We aimed to examine the effects of age, diagnosis year (calendar period) and birth year (cohort) on the incidence trends of breast cancer among Golestan women, Northeast Iran, 2004–2018. Incidence data were obtained by residential status (urban/rural) and ethnic region (Turkmens/non-Turkmens). We calculated age-standardized incidence rates (ASRs) per 100,000 person-years. The estimated annual percentage change (EAPC) were calculated, and age-period-cohort (APC) models fitted to assess non-linear effects of period and cohort as incidence rate ratios (IRRs). The total number of female breast cancer cases in Golestan, 2004–2018, were 3853, with an overall ASR of 31.3. We found higher rates in urban population (40.5) and non-Turkmens region (38.5) compared to rural area (20.8) and Turkmens region (20.2), respectively. There were increasing trends in incidence rates overall (EAPC= 4.4; 95%CI: 2.2, 6.7), with greater changes in rural areas (EAPC=5.1), particularly among non-Turkmens (EAPC=5.8). The results of the APC analysis indicate the presence of significant non-linear cohort effects with increasing IRRs across successive birth cohorts (IRR=0.1 and IRR= 2.6 for the oldest and the youngest birth cohorts vs. the reference birth cohort, respectively). We found increasing trends in breast cancer incidence among Golestan women over the study period, with disparities in patterns and trends by residence area and ethnic region. The observed cohort effects suggest an increasing prevalence of key risk factors for breast cancer in this Iranian population. Further investigations are warranted to clarify the relationships between determinants such as reproductive factors and ethnicity in the region.