Background:Smoking is not, while alcohol consumption is, established as a cause of breast cancer. During the last decade smoking before first childbirth has emerged as a possible risk factor for breast cancer.
Aim:to prospectively examine whether active smoking, especially before first childbirth, increases the risk of breast cancer overall, for non-drinkers of alcohol and if this association differs across ethnic groups.
Methods: We followed 88,300 women, enrolled at age 45-75 years as part of the Multiethnic Cohort Study. Breast cancer was ascertained by linkage to the Surveillance, Epidemiology, and End Results (SEER) Program cancer registries covering the states of Hawaii and California through December 2010. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) while adjusting for relevant confounders.
Results:During a mean follow-up of 15 years, 4,484 women developed invasive breast cancer. Compared with never smokers, smoking characteristics showed consistent dose-response associations with breast cancer risk [age at smoking initiation (ptrend <0.001), smoking duration (ptrend =0.02), number of cigarettes smoked daily (ptrend=0.03) and pack-years (ptrend=0.003)]. Among parous women, those who had smoked six or more years before their first childbirth had an increased risk of breast cancer of 20% overall (HR=1.21, 95% CI: 1.05-1.39) and among non-drinkers only (HR=1,24, 95% CI: 1.01-1.51), compared with never smokers. The test for heterogeneity for the association between ever /never smoking and breast cancer risk by ethnic groups was not significant (p=0.59).
Conclusions:: We found that the increase in breast cancer risk due to smoking is similar for drinkers and non-drinkers of alcohol. Our data does not support any difference for the smoking and breast cancer association for the five examined racial/ethnic groups, although this may be due to lack of power.