Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 Page 6 of 10 Table 3 Age-standardised rate ratios (SRR) of breast cancer, by age group, pooled across all cohorts, for Māori, Pacific and Asian compared to European/Other 25-44 years SRR 95% CI 1.16 (1.06 - 1.27) 1.16 (1.01 - 1.35) 0.86 (0.70 - 1.04) Ethnicity Total Mäori vs European Total Pacific vs European Total Asian vs European Pooled Pooled Pooled women. This “Asian” group is diverse both in terms of country of origin and time in New Zealand, and so is likely to be diverse in terms of reproductive patterns and other risk factors for breast cancer, making the results for Asian women hard to interpret. However despite small numbers, the relatively low rates seen are consistent with the low rates seen in Asian countries. For example IARC’s Cancer Incidence in Five Conti- nents IX reports age standardised rates per 100,000 for those aged 25+ of 52 in India, 67 in China, 45 in Korea, and 56 in Malaysia for the period 1998-2002 (based on available cancer registry data, standardised to WHO World Standard) [2]. The ASR of 126 per 100,000 (95% CI 110-142) found amongst New Zealand Asian women in 2001 - 2004 is considerably higher than rates in Asian countries, which is likely to reflect changes in risk factor distribution following migration. The high incidence of breast cancer amongst Māori women is however not easily explained, as Māori women have a more favourable (known) risk factor profile than European women. Māori women tend to start having children younger and have more children compared to European women [37]. Māori women have lower rates of HT use [40], and similar rates of oral contraceptive use, alcohol consumption and physical activity compared to Figure 2 Trends in age standardized incidence rates of breast cancer by time period and income tertile, age 25+. 45-64 years SRR 95% CI 1.17 (1.10 - 1.24) 0.94 (0.82 - 1.07) 0.66 (0.56 - 0.78) 65+ years SRR 95% CI 1.14 (1.04 - 1.26) 0.77 (0.62 - 0.96) 0.63 (0.48 - 0.83) European women [38,41]. Screening is not contributing to the higher incidence seen, as Māori women have lower uptake of screening [42]. All of this risk factor informa- tion suggests that European/Other women should have higher rates than Māori, yet it is Māori who consistently do so. One partial explanation for the observed higher rates for Māori is post-menopausal obesity among Māori, however it is unlikely to be a full explanation for higher post-menopausal rates. The timing of menarche has previously been postu- lated as a possible reason for the difference in breast cancer rates between Māori and European women [43]. However the evidence to support differences in the tim- ing of menarche and menopause between Māori and non-Māori women is inconclusive and any differences in timing are small [24,40,44]. The prevalence of other postulated breast cancer risk factors such as shift work [45], exposure to particular environmental pollutants [46], or Vitamin D levels [47] may differ between ethnic groups but is unlikely to explain the large differences identified here. Several recent studies have suggested that cigarette smoking initiation shortly after menarche and before first full term pregnancy may increase the risk of breast cancer [48-50]. Early initiation (mean age 11 in 2007) [51] and very high rates of tobacco use amongst young Maori women [52] may be important for breast cancer risk, although the effect of early initiation of smoking on breast cancer risk does not appear to be large enough to explain the patterns seen. There is some evidence that there are differences in tumour biology between ethnic groups. For example, there are differences in tumour grade and estrogen and progesterone receptor (ER and PR) positivity between black, Hispanic and white women in the US [53]. In New Zealand, the potential of genetic differences between ethnic groups has not been thoroughly explored and findings in relation to receptor positivity status have been inconsistent [54,55]. Regardless, while differences in tumour biology may be important with respect to survival differences between women with breast cancer in different ethnic groups [56], it is unclear whether they are important for explaining