Purpose: Our objective was to prospectively examine active smoking and environmental tobacco smoke (ETS) in relation to breast cancer risk, with a focus on exposures during potential windows of susceptibility. Methods: Sister Study cohort participants (n = 50,884) were enrolled between 2003 and 2009 and were followed for a breast cancer diagnosis. Women ages 35–74 in the United States and Puerto Rico were eligible if they had a sister who had been diagnosed with breast cancer. Study participants completed extensive telephone and paper questionnaires including information on established breast cancer risk factors as well as active smoking history and exposure to ETS while in utero and during childhood and adult years. Cox regression analysis was used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs) for invasive breast cancer incidence associated with active smoking and ETS exposure. Results: During follow-up (mean = 6.4 years), 1,843 invasive breast cancers were diagnosed in the study population. Exposure to ETS in adulthood was not associated with increased breast cancer risk. However, nonsmoking women who were exposed to ETS throughout their childhood (18 years) had an 18% higher risk of breast cancer (95% CI, 1.02–1.38) relative to those without any childhood ETS. In utero ETS exposure also was associated with a modest increase in breast cancer incidence (HR = 1.16, 95% CI, 1.01–1.32) among nonsmokers as was paternal smoking prior to the participant's mother's pregnancy (HR = 1.12, 95% CI, 0.98–1.29). Additionally, active smoking prior to first pregnancy for 10 or more pack-years (HR = 1.31, 95% CI, 1.02–1.67) was associated with an elevated risk of breast cancer. Conclusions: In this large, prospective study, we report evidence that both active smoking and ETS exposure during potential windows of susceptibility, including in utero exposure, childhood and prior to first pregnancy, are associated with higher risk of breast cancer.

The following are the 17 highest-scoring abstracts of those submitted for presentation at the 40th Annual ASPO meeting held March 13–15, 2016, in Columbus, OH.