Aim: Higher rates of breast cancer at latitudes further from the equator in comparison to latitudes close to the equator have been shown in several international ecologic studies. Our aim was to investigate the association between breast cancer and latitude in Australia.
Methods: Female breast cancer incidence by five-year age group between 2002 and 2006 were obtained for all mainland Australian Local Government Areas (LGA). Latitude, Accessibility/Remoteness Index of Australia (ARIA) and Index of Relative Socioeconomic Disadvantage (IRSD) were assigned to each LGA. Data were aggregated into five latitude bands (≤27; >27-30; >30-33; >33-36; >36∘S). We estimated the Odds of breast cancer by fitting univariate, bivariable and multivariable logistic regression models with breast cancer as the response variable adjusting for latitude, age-group, IRSD and ARIA. The analysis was replicated with melanoma as the response variable to support the hypothesis that increasing latitude is associated with decreasing sun exposure.
Results: In women aged 20-89 years, the Odds of breast cancer at latitude south of 27ᴼS was 2.1 to 2.4 times higher than at latitude ≤27ᴼS (univariate model, p<.001). In bivariable analysis, age adjusted Odds of breast cancer also increased with increasing latitude for each category of IRSD and remoteness (OR 1.1-2.9, p≤.001). In multivariable analysis, the Odds of breast cancer was 1.22 (1.17-1.26) at latitudes >27-30 and 2.12 (2.04-2.2) to 2.37 (2.28-2.47) at latitudes >30 (all p<.001).Conversely, in the same population, after adjusting for age-group, IRSD and ARIA the Odds of melanoma was 0.62 (0.58-0.63, p<0.001) at latitudes >36ᴼS compared to latitude ≤27ᴼS (p<0.001)
Conclusions: We report a significant relationship between latitude and breast cancer incidence in Australian women which supports international findings showing an ecological gradient for breast cancer. Although we are cannot infer causality, these findings support suggestions that higher vitamin D concentrations associated with increased sunlight exposure is associated with reduced risk of breast cancer.