Does the breast cancer age at diagnosis differ by ethnicity? A study on immigrants to Sweden.

BACKGROUND Age-specific incidence rates for breast cancer in low-risk and high-risk ethnic populations differ by age at which the incidence maximum is reached: around 50 years in low-risk populations and over 60 years in high-risk populations. The interpretation of these differences remains unsettled, one line primarily referring to biological differences, the second one to cohort effects of rapidly increasing rates in young populations, and the third one to incomplete registration of cancer in the elderly. METHODS The nationwide Family-Cancer Database was used to analyze standardized incidence ratios (SIRs) and age at diagnosis of breast cancer in female immigrants to Sweden by their region of origin compared with women native to Sweden matched on birth year and other relevant factors. RESULTS We showed first that the SIRs for breast cancer were lower in many immigrant groups compared with natives of Sweden; women from Turkey had the lowest SIR of 0.45, followed by those from Chile (0.54) and Southeast Asia (0.57). Women from nine regions showed an earlier mean age at diagnosis than their matched Swedish controls, the largest differences being 5.5 years for women from Turkey, 5.1 years for those from Asian Arab and "Other African" countries, 4.3 years for those from Iran, and 4.0 years for those from Iraq. CONCLUSIONS The results show that in many immigrant groups, the diagnostic age is earlier (<50 years) than in natives of Sweden (>50 years), suggesting that true biological factors underlie the differences. These factors may explain much of the international variation in breast cancer incidence. Identifying these factors should advance understanding of breast cancer etiology and prevention.