Incidence-Based Breast Cancer Mortality Trends in Estonia Before and After the Introduction of Organized Mammography Screening: A Register-Based Study

Abstract

Background: Despite the relatively low breast cancer incidence in Estonia, mortality remains high, and participation in mammography screening is below the recommended 70%. The objective of this register-based study was to evaluate incidence-based (IB) breast cancer mortality before and after the introduction of organized mammography screening in 2004. Methods: Breast cancer deaths individually linked to breast cancer diagnosis were obtained from the Estonian Cancer Registry and used for calculating IB mortality. We compared age-specific IB mortality rates across 5-year birth cohorts and 5-year periods. Poisson regression was used to compare IB mortality for one age group invited to screening (50-63) and three age groups not invited to screening (30-49, 65-69, and 70+) during two periods before and after screening initiation (1993-2003 and 2004-2014). Joinpoint regression was used for age-standardized incidence and IB mortality trends. Results: Age-standardized IB mortality has been decreasing since 1997. Age-specific IB mortality for birth cohorts never exposed to screening showed a continuous increase with age, while in cohorts exposed to organized screening the mortality curve flattened or declined after the age of first invitation. Significant decreases in mortality from 1993-2003 to 2004-2014 were seen in the 30-49 (age-adjusted rate ratio 0.51, 95% CI 90.42-0.63) and 50-63 (0.65, 95% CI 0.56-0.74) age groups, while no decline was seen in the 65-69 and 70+ age groups. Conclusions: The age specific IB mortality curves in birth cohorts exposed to screening and the significant mortality decline in the target age group after the initiation of the organized program suggest a beneficial effect of screening. Improved treatment without screening has not reduced mortality in older age groups. Our results support raising the upper screening age limit to 74 years.