Cigarette Smoking and Risk of Breast Cancer in a New Zealand Multi-Ethnic Case-Control Study

Background: The association between breast cancer and tobacco smoke is currently unclear. The aim of this study was to assess the effect of smoking behaviours on the risk of breast cancer among three ethnic groups of New Zealand women. Methods: A population-based case-control study was conducted including breast cancer cases registered on the New Zealand Cancer Registry between 2005 and 2007. Controls were matched by ethnicity and 5-year age-group. Logistic regression was used to estimate the association between breast cancer and smoking at different time points across the lifecourse, for each ethnic group. Estimated odds ratios (OR) were adjusted for established risk factors. Results: The study comprised 1,799 cases (302 Maori, 70 Pacific, 1,427 non-Maori/non-Pacific) and 2,540 controls (746 Maori, 191 Pacific, 1,603 non-Maori/non-Pacific). There was no clear association between smoking and breast cancer for non-Maori/non-Pacific women, although non-Maori/non-Pacific ex-smokers had statistically significant increased risk of breast cancer when smoking duration was 20 years or more, and this remained significant in the fully adjusted model (OR 1.31, 95% CI 1.03 to 1.66). Maori showed more consistent increased risk of breast cancer with increasing duration among current smokers (<20 years OR 1.61, 95% CI 0.55 to 4.74; 20+ years OR 2.03, 95% CI 1.29 to 3.22). There was a clear pattern of shorter duration since smoking cessation being associated with increased likelihood of breast cancer, and this was apparent for all ethnic groups. Conclusion: There was no clear pattern for cigarette smoking and breast cancer incidence in non-Maori/non-Pacific women, but increased risks were observed for Maori and Pacific women. These findings suggest that lowering the prevalence of smoking, especially among Maori and Pacific women, could be important for reducing breast cancer incidence.