Investigating reasons for ethnic inequalities in breast cancer survival in New Zealand

Objective. This study investigated the role that demographic and tumour factors play in explaining ethnic inequalities in breast cancer survival. Design. Breast cancer cases notified to the New Zealand Cancer Registry (NZCR) from April 2005 to April 2007 were followed up to April 2009. Maori, Pacific and non-Maori/non-Pacific women were categorised according to ethnicity on the NZCR. Deprivation was analysed as quintiles of the New Zealand area-based index of socio-economic position. Relative survival rates were estimated using ethnic-specific life tables. Missing values were imputed and excess mortality modelling was used to estimate the contribution of demographic and tumour factors to ethnic inequalities in survival. Results. There were 2968 breast cancer cases (76.5% non-Maori/non-Pacific, 17% Maori, and 6.5% Pacific) included and 433 recorded deaths. Relative survival rates at 4 years were 91.5% (95% confidence interval (CI) 89.7 to 92.9) for non-Maori/non-Pacific, 86.2% (CI 80.3 to 90.4) for Maori, and 79.6% (CI 68.2 to 87.2) for Pacific women. Using non-Maori/non-Pacific as the reference group, the age-adjusted hazard ratio (HR) dropped for Maori from 1.76 (CI 1.22 to 2.48) to 1.43 (CI 0.97 to 2.10) when further adjusted by deprivation. For Pacific the HR dropped from 2.49 (CI 1.57 to 3.94) to 1.94 (CI 1.20 to 3.13). Inequalities persisted after adjustment for subtype variables (ER/PR/HER2), but adjusting for access to care variables (extent/size) eliminated the ethnic inequalities in excess mortality. Conclusion. Ethnic disparities in breast cancer survival in New Zealand can be attributed to deprivation and differential access to health care rather than differences in breast cancer subtypes.