Background: Breast cancer is more common in Maori than in non-Maori women under the age of 40 years and is equally common in older women, despite Maori being generally of lower socioeconomic status and having had a higher fertility rate than non-Maori.
Methods: Data from a nationwide population-based case-control study of breast cancer in New Zealand women aged 25-54 years were used to compare the age-adjusted distribution of reproductive and other risk factors for breast cancer in self-identified Maori and non-Maori women from the control group. Separate analyses also were carried out for women aged 25-39 years and for those aged 40-54 years. The risk of breast cancer according to the proportion of Maori ancestry was estimated using multiple logistic regression simultaneously adjusting for several risk factors.
Results: Significant differences were found between self-identified Maori and non-Maori women in the age-adjusted frequencies for education level, socioeconomic status, age at first full-term pregnancy, parity, and duration of breastfeeding; the profile in all instances suggesting a lower risk of breast cancer for Maori than for non-Maori. There were no significant differences with respect to age at menarche, surgery for benign breast disease or a family history of breast cancer. Significantly more Maori than non-Maori were in the highest quartile of recent body mass index. Women self-identified as Maori has an approximately twofold higher risk of breast cancer than non-Maori women.
Conclusions: Maori have high rates of breast cancer despite having a more favourable profile than non-Maori for most identified risk factors.
PIP: National statistics collected in New Zealand since the mid-1960s have identified higher rates of breast cancer in Maori women under 40 years of age than their non-Maori counterparts, despite their low socioeconomic status and high fertility. Data from a nationwide population-based case-control study of breast cancer in New Zealand women 25-54 years of age were used to compare the age-adjusted distribution of reproductive and other risk factors for breast cancer in self-identified Maori (n = 89) and non-Maori women (n = 1771) from the control group. Compared with women with no Maori ancestors, women 25-39 years old with at least half Maori ancestry had a two-fold higher risk of breast cancer after adjustment for known risk factors (odds ratio, 2.2; 95% confidence interval, 1.2-4.2). However, when data from the control group were analyzed, Maori women had a significantly more favorable profile in terms of breast cancer risk than their non-Maori counterparts in terms of education level, socioeconomic status, age at first full-term pregnancy, parity, and duration of breast feeding. The only exception to this pattern was body mass index. 62.1% of Maori controls 25-54 years old, compared with 23.1% of their non-Maori counterparts, were in the highest quartile of recent body mass index (p 0.001). The excess of breast cancer in young Maori may reflect unknown genetic factors that increase susceptibility.