Adult Physical Activity and Breast Cancer Risk in Women with a Family History of Breast Cancer.

BACKGROUND: Recreational physical activity has been consistently associated with reduced breast cancer risk. Less is known about how family history of breast cancer affects the association and whether it varies by menopausal status.
METHODS: The Sister Study is a cohort of 50,884 women who had a sister with breast cancer but no prior breast cancer themselves at enrollment. Women reported all recreational sport/exercise activities they participated in over the past 12 months. Hours/week and MET-hours/week of physical activity were considered in association with breast cancer risk. Hazard ratios (HR) and 95% confidence intervals (CI) were calculated with Cox regression. Extent of family history, examined as a modifier, was characterized by a Bayesian score incorporating characteristics of the family structure.
RESULTS: During follow-up (average 8.4 years), 3,023 cases were diagnosed. Higher hours/week (HR≥7vs<1 = 0.77; 95% CI, 0.66-0.90) and MET-hours/week (HRquartile4vs1 = 0.75; 95% CI, 0.67-0.85) of physical activity were associated with reduced postmenopausal breast cancer risk. Hours/week and MET-hours/week were associated with suggestively increased premenopausal breast cancer risk (MET-hours/week HRquartile4vs1 = 1.25; 95% CI, 0.98-1.60). Associations did not vary with extent of family history. However, the increased risk in premenopausal women may be limited to those with stronger family history.
CONCLUSIONS: In women with a family history of breast cancer, physical activity was associated with reduced postmenopausal, but not premenopausal, breast cancer risk and was not modified by extent of family history. IMPACT: This was the first study to examine the association between physical activity and breast cancer risk in a large population with a family history of breast cancer. ©2018 American Association for Cancer Research.