Ethnic differences in breast cancer outcomes in Aotearoa New Zealand

Abstract

Background: Indigenous Māori women experience significantly worse breast cancer outcomes compared with European women in New Zealand. Underlying reasons for this disparity are complex, and inadequately explained in the existing literature. This study was aimed at estimating the Māori-NZ European breast cancer survival disparity, and to identify and quantify impacts of various factors contributing to this disparity. Methods: Data for all women with newly diagnosed invasive breast cancer in the Waikato District Health Board area between 01/01/1999 and 31/12/2012 were obtained from the Waikato Breast Cancer Register, and through a retrospective patient clinical notes review. Patient, tumour and treatment characteristics of Māori and NZ European women were compared in adjusted multivariable models. Cancer specific survivals were compared using Kaplan-Meier survival curves, while contributions of different factors towards the survival disparity were quantified with Cox proportional hazard modelling. Results: Of the total of 2791 women included, 2260 (80.1%) were NZ European and 419 (15%) were Māori. Compared with NZ European women, Māori were significantly more likely to be diagnosed with more advanced breast cancer, to have comorbidities and to experience longer treatment delays. Māori were significantly less likely to be diagnosed through screening, to receive adjuvant radiotherapy and endocrine therapy based on recommended guidelines, and to be optimally adherent with endocrine therapy. Compared with NZ European women, Māori had a significantly higher age adjusted cancer specific mortality (HR=2.02, 95% CI, 1.59-2.58) with significantly lower 5-year (86.8% vs. 76.1%, p<0.001) and 10-year (79.9% vs. 66.9%, p<0.001%) crude cancer-specific survival rates. Stage at diagnosis explained approximately 40% of the survival disparity while screening, treatment and patient factors (i.e. comorbidity, obesity and smoking) contributed by approximately 15% each. The final model accounted for almost all the cancer survival disparity between Māori and NZ European women (HR=1.07, 95% CI, 0.80-1.44). Conclusions: Māori women with breast cancer are twice as likely as NZ European women to die from their cancer. Lower screening coverage, delay in diagnosis, inferior quality of treatment and greater patient comorbidity were largely responsible for this survival disparity. Improving healthcare access and provision of an equitable cancer care for Māori needs greater attention.