Ethnic and socioeconomic trends in breast cancer incidence in New Zealand

Background: Breast cancer incidence varies between social groups, but differences have not been thoroughly examined in New Zealand. The objectives of this study are to determine whether trends in breast cancer incidence varied by ethnicity and socioeconomic position between 1981 and 2004 in New Zealand, and to assess possible risk factor explanations. Methods: Five cohorts of the entire New Zealand population for 1981-86, 1986-1991, 1991-1996, 1996-2001, and 2001-2004 were created, and probabilistically linked to cancer registry records, allowing direct determination of ethnic and socioeconomic trends in breast cancer incidence. Results: Breast cancer rates increased across all ethnic and socioeconomic groups between 1981 and 2004. Maori women consistently had the highest age standardised rates, and the difference between Maori and European/Other women increased from 7% in 1981-6 to 24% in 2001-4. Pacific and Asian women had consistently lower rates of breast cancer than European/Other women over the time period studied (12% and 28% lower respectively when pooled over time), although young Pacific women had slightly higher incidence rates than young European/other women. A gradient between high and low income women was evident, with high income women having breast cancer rates approximately 10% higher and this difference did not change significantly over time. Conclusions: Differences in breast cancer incidence between European and Pacific women and between socioeconomic groups are explicable in terms of known risk factors. However no straightforward explanation for the relatively high incidence amongst Maori is apparent. Further research to explore high Maori breast cancer rates may contribute to reducing the burden of breast cancer amongst Maori women, as well as improving our understanding of the aetiology of breast cancer.