Association between latitude and breast cancer incidence in mainland Australian women

Aim: To investigate whether breast cancer incidence increases with increasing latitude in mainland Australian women. Methods. A cross-sectional study of female breast cancer and cutaneous melanoma incidence 2002–2006 by 5-year age group and local government area. Latitude, Accessibility/Remoteness Index of Australia (ARIA), and Index of Relative Socioeconomic Disadvantage (IRSD) were assigned to local government areas. Latitude was grouped into bands (≤27°S; >27–30°S; >30–33°S; >33–36°S, and >36°S), and IRSD was divided into quintiles and ARIA into four categories. Breast cancer rates were age standardized using the direct method. The joint effects of latitude, age, IRSD, and ARIA on incidence of breast cancer and cutaneous melanoma were assessed using multiple logistic regressions. Results. At latitudes south of 30°S, rates of breast cancer were over double that north of 27°S (76.4 versus 160.2–176.5). Age-adjusted odds ratios of breast cancer were increased in all latitudes south of 30°S compared with north of 27°S within each IRSD and ARIA category (all P<0.001). After adjusting for age, IRSD, and ARIA, the odds ratio of breast cancer south of 30°S was 1.92 (95% CI 1.84–2.09; P<0.001), whereas cutaneous melanoma was 0.65 (95% CI 0.61–0.68; P<0.001) times north of 30°S. Discussion. Increasing latitude is positively associated with breast cancer and negatively associated with cutaneous melanoma incidence. These findings support suggestions that increased risk of breast cancer might be explained by lower ultraviolet radiation-induced vitamin D synthesis.