Active cigarette smoking and risk of breast cancer.

Although epidemiological evidence on the role of active cigarette smoking in breast cancer risk has been inconsistent, recent literature supports a modest association between smoking and breast cancer. This association is particularly observed in women who smoke for a long duration, or who smoke for a long time prior to their first pregnancy. Here, we provide updated results on cigarette smoking and breast cancer risk in the Canadian National Breast Screening Study (NBSS). The NBSS is a large cohort of 89,835 women, aged 40-59, who were followed for a mean of 22.1 years, resulting in the ascertainment of 6,549 incident cases of breast cancer. Cox proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) for the association of cigarette smoking variables with breast cancer risk. We found breast cancer to be associated with duration (40 years vs. 0: HR = 1.57; 95%CI = 1.29-1.92), intensity (40 cigarettes per day vs. 0: HR = 1.21; 95%CI = 1.04-1.40), cumulative exposure (40 pack-years vs. 0: HR = 1.19; 95%CI = 1.06-1.13) and latency (40 years since initiation vs. 0: HR = 1.19; 95%CI = 1.10-1.53) of cigarette smoking. Number of years smoked prior to first full-term pregnancy was associated with higher risk of breast cancer than comparative years smoked post-pregnancy (among parous women, 5 years pre pregnancy vs. 0: HR = 1.18; 95%CI = 1.10-1.26). These results strongly support a role for cigarette smoking in breast cancer etiology and emphasize the importance of timing of this exposure.