The association between greenness, walkability, and post-menopausal breast cancer risk in Alberta’s Tomorrow Project

• Greener and more walkable neighbourhoods are often associated with reduced chronic disease incidence • There are few studies evaluating the association between greenness and walkability and breast cancer risk • This study found no consistent association between walkability and breast cancer risk • This study found a non-significant decreased risk of breast cancer in areas with high levels of green space Breast cancer is the most prevalent cancer in women, with post-menopausal diagnoses more common than pre-menopausal. Risk factors for breast cancer include genetics and modifiable factors related to diet, behavior, and the environment. The objective of this study was to assess the impact of greenness and walkability on post-menopausal breast cancer risk. Longitudinal cohort study design utilizing the Alberta Tomorrow Project cohort, with 15,536 post-menopausal women and 523 incident breast cancer cases over 12.6 years of follow-up. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using Cox proportional hazard regression. The adjusted HR for an interquartile range (IQR = 0.1) increase in greenness measured within 1000 meters of the home location was 0.97 (95% CI = 0.88, 1.07). Adjusted HRs for walkability ranged from 1.07 (95% CI =0.77, 1.39) to 1.22 (95% CI =0.90-1.60) for quintiles 2 through 5 (most walkable) in relation to quintile 1 (least walkable). In this study the least walkable areas had lower breast cancer risk, after adjusting for social deprivation, age, and body mass index, although results were statistically non-significant except for an increased risk in quintile 4 (more walkable). Higher average greenness within 1000 meters of the home was not significantly associated with postmenopausal breast cancer risk after adjusting for smoking and social deprivation. More research in other settings is needed to advance knowledge on the association between the built environment and breast cancer risk.