Ethnic differences in breast cancer risk and survival: A study on immigrants in Sweden

Background. There are large geographic differences in breast cancer risk but whether survival differs between low- and high-risk groups is less well-established. As the survival of cancer depends on the level of healthcare and awareness of disease risks, subtle differences in cancer biology cannot be revealed in international comparisons. Instead, comparison of diverse immigrant groups in a country of uniformly accessible healthcare system should enable conclusions to be made about ethnic determinants of cancer risk and survival. Material and methods. The Swedish Family-Cancer Database was used to calculate standardized incidence (SIRs) and hazard ratios (HRs) of death from female breast cancer in 12 505 and 137 547 patients diagnosed with breast cancer among immigrants and Swedes, respectively. The ratios were adjusted for age, period, region, parity, and age at first childbirth. Ordinal logistic regression analysis was used to estimate odds ratios (ORs) for the clinical TNM classes. The analyses were stratified by menopausal status and histology. Results. Turks, Southeast Asians, and Chileans had the lowest breast cancer risk (SIR = 0.44; 95% CI 0.37-0.51) and Iraqis the highest risk (1.19; 1.05-1.35), mainly due to premenopausal cancer (1.51; 1.27-1.78). The HRs for all breast cancers were between 0.98 (0.81-1.18) (low-risk Europeans) and 1.24 (0.94-1.63) (lowest-risk non-Europeans), but were not significant. No differences in survival of ductal carcinoma between immigrants and Swedes were found, while low-risk non-Europeans had a HR of 2.88 (1.37-6.08) for lobular carcinoma. Low-risk non-Europeans were diagnosed in a higher T-class (OR = 1.87; 1.21-2.87) than Swedes. Conclusion. We did not find any evidence that ethnic differences in breast cancer risk substantially affect the survival. The observed poor survival of some low-risk immigrants in lobular carcinoma may be related to treatment. The tendency of low-risk immigrants to present with higher T-class compared to Swedes may depend on their lower participation in the mammography screening program.