Case-control study of tobacco smoke exposure and breast cancer risk in Delaware

Background: Tobacco smoke exposure may be associated with increased breast cancer risk, although the evidence supporting the association is inconclusive. We conducted a case-control study in Delaware, incorporating detailed exposure assessment for active and secondhand smoke at home and in the workplace. Methods: Primary invasive breast cancer cases diagnosed among female Delaware residents, ages 40-79, in 2000-2002 were identified through the Delaware cancer registry (n = 287). Delaware drivers license and Health Care Finance Administration records were used to select age frequency-matched controls for women <65 and ≥ 65, respectively. Detailed information on tobacco smoke exposure was obtained through telephone interviews. Results: A statistically significant increased risk of breast cancer was observed for ever having smoked cigarettes (odds ratio = 1.43, 95% confidence interval = 1.03 - 1.99). However, there was no evidence of a dose-response relationship between breast cancer risk and total years smoked, cigarettes per day, or pack-years. Neither residential nor workplace secondhand smoke exposure was associated with breast cancer. Recalculations of active smoking risks using a purely unexposed reference group of women who were not exposed to active or secondhand smoking did not indicate increased risks of breast cancer. Conclusion: These findings do not support an association between smoking and breast cancer.