Worldwide Age at Onset of Female Breast Cancer: A 25-Year Population-Based Cancer Registry Study.

A higher frequency of early onset female breast cancers (BC) has been observed in low/middle income countries than in high income countries. We quantified the role of population ageing to this pattern using data from all population-based cancer registries (CRs) worldwide. Patients' median age at BC onset and that of the general population were extracted for CRs listed in volumes VI (1983-1987 years) through XI (2008-2012 years) of Cancer Incidence in Five Continents. Their association was assessed at cross-sectional level by linear regression model and longitudinally considering 25-year ageing of the population in long-standing CRs listed at the beginning and at the end of the study. During 2008-2012, each one-year increase of population ageing was associated with a nearly ½ year increase of age at BC diagnosis. Population demographics explained forty-two percent of the age variance for BC. In 1983-1987, long-standing CRs with a median age at BC below age 61.8 years showed an increase of age at BC after 25-years. Worldwide, age at BC diagnosis essentially reflected the median age of the population. Changes in BC detection methodology likely lessened this association. Nevertheless, the elevated absolute number of BCs in young populations deserves strategies of BC prevention.