Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 ‘Asian’ ethnic group, mostly from a recent wave of migrants [30-32]. By the 2006 census the New Zealand population was just over 4 million, with 65% European, 14% Māori, 7% Pacific, 9% Asian, and around 11% iden- tifying with other ethnic groups [33]. A modified total ethnicity approach was used for this work. Total ethnicity places an individual in all ethnic groups that they identify with, thus capturing (most) multiple ethnic affiliations of individuals [34]. If indivi- duals indicated any/all of Māori, Pacific and/or Asian ethnic affiliation they were placed in any/all of Total Māori, Total Pacific, Total Asian ethnic groups. The residual people who did not indicate any of the above ethnic affiliations were placed in the non-Māori/Pacific/ Asian (nMPA). For simplicity this category is referred to as European/Other in this paper. The 1981 census ques- tion was based on ethnic origin rather than ethnic affiliation and blood quantum measures were used. In order to convert this into total ethnicity to be consistent with later years, we classified someone as Māori if they recorded any fraction as Māori, and likewise for Pacific and Asian. Household equivalised income was calculated and assigned to each individual within the household. Perso- nal incomes were CPI adjusted to 2001, summed over the household and adjusted for number of individuals within the household, using the New Zealand-specific Jensen equivalisation index [35]. Equivalised incomes were then attached to each individual. In order to create income tertiles for analyses, individuals were grouped into 5 year age groups pooled across cohorts, and then ranked and divided into tertiles within each of these five-year age groups. Analyses were also performed using education as a measure of socioeconomic position, and are available from the authors on request. Outcome Cancer outcome was assessed by having a cancer regis- tered on the NZ Cancer Registry. All breast cancers (ICD-10 code C50) were included. Ductal carcinoma in situ (DCIS) was excluded. Cancers prior to 2000 were forward mapped from ICD-9 to ICD-10 codes using mapping codes provided by New Zealand Health Infor- mation Service. Analyses For the analyses presented here, data were restricted to adults aged 25 and over who were in their usual resi- dence on census night. All analyses were conducted in SAS v8.2 or v9. Person time was censored after development of the first breast cancer. Incidence rates, rate ratios and rate differences (and 95% confidence intervals) were calcu- lated after direct standardisation of the cohorts to the Page 3 of 10 age structure of the WHO World Standard population [36], and to the ethnic structure of the 2001 New Zealand population for trends by income (because ethni- city may act as a confounder on the income - breast cancer relationship). All analyses were performed on complete data only, with no imputation of missing data. Statistical tests of trend were conducted for rates, and rate differences, and of the log transformed rate ratios. All measures were also calculated for all five cohorts pooled [28]. Results Table 1 shows the numbers of cancers and total person time, in each of the ethnic and socioeconomic groups in each cohort. Ethnic trends Figure 1 and Table 2 show age standardised breast cancer rates for Māori, Pacific, Asian and European/ other women in each time period. Rates for Māori women increased seventy percent in the time period studied, compared to about 50% for European/other (p for trend both < 0.01). Incidence rates in Pacific and Asian women increased by 25% and 80% respectively. Both the relative and absolute differences between Māori and European/other women increased between 1981 and 2004 (Table 2). Māori women had 7% higher rates than European/other women in 1981-86, and 24% higher rates in 2001-4 (SRR 1.07 in 1981-86 and 1.24 in 2001-04), and the absolute difference increased from 9 to 40 per 100,000 women. By contrast Asian and Pacific women had consistently lower breast cancer rates than European/Other women and the differences between Pacific and Asian and European/Other women did not change markedly over time. Table 3 shows the patterns of differences between Māori, Pacific and European/Other ethnic groups at differ- ent ages, pooled for all cohorts to increases statistical pre- cision and stability when assessing heterogeneity by age. Compared to European/Other women, Māori women had a significant 15-17% increased risk across all age groups. The rate ratio for Pacific women varied with age with young Pacific women having an increased risk (SRR 1.17; 95% CI 1.01-1.35) and older women a decreased risk (SRR 0.77; 95% CI 0.62-0.96) compared to European/Other women. Asian women had a lower risk than European/ Other women across all ages, with a decreasing risk by age relative to European/Other women. Socioeconomic trends Figure 2 shows age and ethnicity standardised breast cancer incidence rates for low, medium and high income women for each cohort. A gradient with income is clearly seen in 1986-91, 1996-01 and 2001-04, with