Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 Page 4 of 10 Table 1 Number of cancers* and women-years of observation (25 + years) across five time periods according to ethnicity and income 1981-86 1986-91 1991-96 1996-2001 2001-04 Cancers Person years Cancers Person years Cancers Person years Cancers Person years Cancers Person years (n) (n) (n) (n) (n) Ethnicity Total Mäori Total Pacific Total Asian European/Other Miss Eth Income Low Income Medium Income High Income Missing Income 348 87 30 5,229 132 1,911 1,551 1,449 909 367,781 105,369 50,527 3,957,509 70,116 1,404,919 1,251,068 1,099,996 790,545 486 162 60 6,825 60 2,235 2,520 1,878 942 424,739 138,460 71,662 4,272,857 60,745 1,545,090 1,533,130 1,098,036 783,240 687 168 111 7,509 30 2,250 3,063 2,115 1,065 485,574 189,060 145,207 4,472,644 45,976 1,508,029 1,643,443 1,369,292 807,564 1,044 288 264 8,748 96 2,799 3,240 2,805 1,557 628,063 238,991 261,012 4,620,738 70,897 1,582,935 1,648,529 1,561,314 998,890 900 255 351 7,638 90 2,523 2,436 2,748 1,512 484,431 205,681 282,592 3,587,142 52,834 1,244,643 1,107,475 1,353,848 887,079 *All count data are random rounded to a near multiple of 3, with a minimum cell size of 6, as per Statistics New Zealand protocol high income women having consistently higher rates, medium income women intermediate rates, and low income women the lowest rates. Over time, rates increase by about 50% in all income groups. Table 4 shows the age and ethnicity standardised rates for each income group, and the relative and absolute differences of medium and low compared to high income women for each cohort. The differences were relatively stable over time, with no definite increase or decrease in the differences between income groups over the time period studied. Overall low income women had rates approximately 10% lower than high income women. Discussion This study demonstrates a gradient of increasing breast cancer incidence with increasing income, stable Figure 1 Trends in age standardized incidence rates of breast cancer by time period and ethnic group, age 25+. over the time period studied. By ethnicity, lower breast cancer incidence rates were found amongst Pacific and Asian women compared to European/other women. Young Pacific women, however, had relatively high rates. Higher and more rapidly increasing rates were found amongst Māori women across all age groups. The relatively high rates found amongst women of higher income levels are consistent with international patterns, and with local information about the distribu- tion of known risk factors. The patterns seen in Pacific and Asian women are also consistent with what is known about the distribution of risk factors. High rates amongst Māori women are, however, a surprising find- ing given what is known about reproductive patterns amongst Māori and the distribution of other known risk factors. Investigation of the high and steeply increasing rates amongst Māori women is important for reducing the burden of disease in this group, but also may pro- vide important clues to improved understanding of the aetiology of breast cancer; we offer some potential explanations to test below. Ethnic differences and trends Breast cancer rates in European women in New Zealand, which rose from 114 to 170/100,000 women per year between 1981-86 and 2001-04 (for women 25+), are comparable to breast cancer rates in European women in other developed countries. For example, in the US non-Hispanic white women have an annual age standar- dised rate of 189/100,000 for women 25+, (based on 14 SEER registries, 1998-2002; using the same WHO World Standard) [2]. In Scotland women 25+ have an annual age standardised rate of 152/100,000 (1998-2002; using WHO World Standard) [2].