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

Abstract 

Background: This study investigated the role that demographic and tumour factors play in explaining socioeconomic inequalities in breast cancer survival. Methods: Breast cancer cases notified to the New Zealand Cancer Registry (NZCR) from April 2005 to April 2007 were followed up to April 2009. The New Zealand area-based deprivation index (NZDep) was used as a measure of socioeconomic position. Relative survival rates were estimated using sex-, deprivation- and ethnic-specific life tables. Multiple imputation was used to impute missing data. Excess mortality modelling was used to estimate the contribution of demographic and tumour factors to inequalities in survival. Results: There were 2968 breast cancer cases included and 433 recorded deaths. Relative survival rates at 4 years varied across deprivation groups. Using NZDep deciles 1–4 (least deprived) as the reference group, the age- and ethnicity-adjusted hazard ratio (HR) for NZDep deciles 7–8 was 2.03 (CI 1.36–3.04) and for NZDep deciles 9–10 was 1.93 (CI 1.28–2.92). In the fully adjusted model there remained 50% excess mortality for the two most deprived groups compared to the most affluent. Variables which measured timely access to care (extent/size) accounted for more of the survival disparity than breast cancer subtype variables (ER/PR/HER2). Conclusion: Women from deprived areas in New Zealand who are diagnosed with breast cancer are less likely to survive as long as those from affluent areas. A substantial proportion of these socioeconomic disparities can be attributed to differential access to health care although other factors, currently unknown, are also likely to play an important role.

Keywords: Breast cancer, Survival, Socioeconomic inequalities, Multiple imputation, Excess mortality