Background: The age-standardised incidence of breast cancer varies geographically, with rates in the highest-risk countries more than five times those in the lowest-risk countries.
Methods: We investigated the correlation between male (MBC) and female breast cancer (FBC) incidence stratified by female age-group (<50 years, and ≥50 years) and used Poisson regression to examine male incidence rate ratios according to female incidence rates.
Results: Age-adjusted breast cancer incidence rates for males and females share a similar geographic distribution (Spearman's correlation=0.51; P<0.0001). A correlation with male incidence rates was found for the entire female population and for women aged 50 years and over. Breast cancer incidence rates in males aged <50 years were not associated with FBC incidence, whereas those in males aged 50 years were. MBC incidence displays a small 'hook' similar to the Clemmesen's hook for FBC, but at a later age than the female hook.
Interpretation: Further investigation of possible explanations for these patterns is warranted. Although the incidence of breast cancer is much lower in men than in women, it may be possible to identify a cause common to both men and women.