Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 R E S E A R C H A R T I C L E Open Access Ethnic and socioeconomic trends in breast cancer incidence in New Zealand Ruth Cunningham*, Caroline Shaw, Tony Blakely, June Atkinson, Diana Sarfati Abstract 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. Māori women consistently had the highest age standardised rates, and the difference between Māori 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 Māori is apparent. Further research to explore high Māori breast cancer rates may contribute to reducing the burden of breast cancer amongst Māori women, as well as improving our understanding of the aetiology of breast cancer. Background Breast cancer is the most commonly diagnosed non-skin cancer and the leading cause of cancer death for New Zealand women [1]. Rates of breast cancer in New Zealand are similar to other developed countries, with an age-standardised rate of around 80 per 100,000 women [2]. The incidence of breast cancer has been increasing in New Zealand, with rates nearly doubling in the second half of last century. This increase is likely to be due to changes in both risk factor patterns and methods of diagnosis [3,4]. Breast cancer incidence varies between social groups. In general groups with higher income and education have higher rates of breast cancer [3,4]. White ethnic groups * Correspondence: Ruth.Cunningham@otago.ac.nz Department of Public Health, University of Otago Wellington, PO Box 7343, Wellington 6242, New Zealand tend to have higher rates of breast cancer than non-white groups in the same areas [5-7]. These differences in rates are thought to relate to differences in risk factor distribu- tion between socioeconomic and ethnic groups [8-10]. Epidemiological studies suggest that reproductive fac- tors play a major role in determining breast cancer risk. Older age at first birth, lower parity, lack of breast feed- ing, younger age at menarche, older age at menopause and use of menopausal hormone therapy (HT) are all related to increases in breast cancer risk [11], and can all vary between social groups [12]. Other important modifiable risk factors that may be relevant to differ- ences between social groups include physical inactivity, post menopausal obesity, and regular alcohol consump- tion [11]. Environmental and genetic differences between groups may also be important. Breast cancer screening will also increase incidence [13,14], and may © 2010 Cunningham et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.