Abstract 2917: The association of smoking before first childbirth with breast cancer risk is independent from alcohol use and does not seem to differ across ethnic groups: the multiethnic cohort study
Authors: Gram Inger T. , Park Song-Yi , Little Melissa A. , Kolonel Laurence N. , Maskarinec Gertraud , Wilkens Lynne R. , Hendereson Brian E. , Marchand Loic L. 
Published: May 2015

Abstract
Background; The association of smoking with breast cancer is still the subject of scientific inquiry. In early studies, women often reported starting to smoke after their first live childbirth, while in the most recent cohort studies the women started before giving birth. In the former studies smokers had no increased risk of breast cancer, while the latter studies report that smokers have an increased risk of breast cancer. The purpose of the present study was to prospectively examine whether active smoking increases the risk of breast cancer overall and among non-drinkers of alcohol, as well as according to onset of smoking in relation to first childbirth, and according to race/ ethnicity in the MEC, a mature cohort study that enrolled older women of five ethnic groups who are predominately non-drinkers of alcohol.
Methods: We followed 88,374 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 2007. 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 12 years, 3,821 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 =0.002), smoking duration (ptrend =0.03), number of cigarettes smoked daily (ptrend=0.01) and pack-years (ptrend=0.003)]. The patterns for non-drinkers were quite similar to those for women overall, although with the smaller sample sizes, only the test for trend for total lifetime use (pack-years) was statistically significant (ptrend =0.02). Among parous women, those who had smoked three or more years before their first live childbirth had an increased risk of breast cancer of 20% overall (95% CI 1.07-1.36) and among non-drinkers only ( 95% CI 1.01-1.43), compared with never smokers. For parous women, both Native Hawaiians (ptrend =0.02) and whites (ptrend =0.03) showed a positive dose-response relationship between duration of smoking before the first childbirth and breast cancer risk. The test for heterogeneity for the association between ever /never smoking and breast cancer risk by ethnic groups was not significant (p=0.49).
Conclusions: We found that the smoking-related increase in breast cancer risk is similar for drinkers and non-drinkers of alcohol. Our data do 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.
Citation Format: Inger T. Gram, Song-Yi Park, Melissa A. Little, Laurence N. Kolonel, Gertraud Maskarinec, Lynne R. Wilkens, Brian E. Hendereson, Loic Le Marchand. The association of smoking before first childbirth with breast cancer risk is independent from alcohol use and does not seem to differ across ethnic groups: the multiethnic cohort study. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2917. doi:10.1158/1538-7445.AM2014-2917