Abstract 661: Physical activity, sitting time, and postmenopausal breast cancer risk by histopathologic characteristics in a large US cohort

Abstract

Abstract There is convincing evidence that physical activity is associated with lower risk of postmenopausal breast cancer, likely through its impact on circulating sex hormones, improved insulin sensitivity, and/or weight control. However, the relationship between time spent sitting and breast cancer is not well-defined. Furthermore, it is unclear whether the relationship of physical activity or sitting time with postmenopausal breast cancer varies by histopathologic characteristics of the tumor (defined by stage, histology, or estrogen/progesterone receptor (ER/PR) status), use of postmenopausal hormones (PMH), or body mass index (BMI). Therefore, we examined recreational physical activity and leisure-time spent sitting in relation to breast cancer risk among 73,608 postmenopausal women who were cancer-free and enrolled in the American Cancer Society Cancer Prevention Study II (CPS-II) Nutrition Cohort, a prospective study of cancer incidence and mortality that began in 1992. From 1992-2007, 4,622 incident breast cancer cases were identified. Cox proportional hazards regression modeling was used to compute rate ratios (RR) and 95% confidence intervals (CI) while adjusting for potential confounders including age, race, education, BMI, PMH use, and other breast cancer risk factors. Women who engaged in the highest amount of physical activity (>42 MET-hours/week) had 22% lower risk of breast cancer compared to women with the least activity (0-<7 MET-hours/week) (RR 0.78, 95% CI 0.63-0.95; trend p=0.09). Women who walked an hour per day were 15% less likely to develop breast cancer during follow-up compared to women who walked fewer than 3 hours per week (95% CI 0.76-0.95). Sitting time was not associated with breast cancer risk (RR=0.98, 95% CI 0.89-1.09 for 6+ vs <3 hours/day spent sitting). When examining associations by stage, the association between physical activity and breast cancer risk was only observed for localized breast cancer (RR=0.67, 95% CI 0.51-0.89; trend p=0.03), and not for in situ or regional/distant stage. The association did not differ by histology or ER/PR status. The association between physical activity and risk was not modified by BMI (interaction p=0.73) or by PMH (interaction p=0.74), although the lower RR associated with the highest level of physical activity was noted only in non-users of PMH. These findings provide further evidence that physical activity is associated with lower risk of postmenopausal breast cancer and that the benefits of physical activity do not appear to be limited to any specific subgroup of women. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 661. doi:1538-7445.AM2012-661