Abstract P1-07-20: The smoking related risk of breast cancer and proportion of avoidable breast cancer cases due to passive and active smoking in middle-aged women in Norway in 2012: The Norwegian women and cancer study 1991-2012
Authors: Gram IT , Little MA , Lund E , Braaten T 
Published: May 2016

Abstract
Background: The burden of smoking on society may be underestimated as previous estimates of cancer due to smoking have generally not included breast cancer. We utilized the Norwegian Women and Cancer Study, a nationally representative prospective cohort study to examine the risk of breast cancer due to passive and active smoking. We also estimated the proportion of breast cancer attributable to passive and active smoking.
Material and methods: Our study included 130053 women, aged 34 to 70 years, who completed a baseline questionnaire between 1991 and 2007. We followed the women through linkages to the Cancer Registry of Norway and the Norwegian Central Population Register, to identify all cancer cases, emigrations, and deaths, respectively, using the unique national 11-digit personal identification number. Person-years were calculated from the start of follow-up to the date of any incident cancer diagnosis, emigration, death, or the end of follow-up December 31, 2012, whichever came first. Breast cancer cases were classified according to the original codes in the International Classification of Diseases, Seventh Revision including estrogen and progesterone hormone tumor receptor status. We used Cox proportional hazards models, adjusting for relevant confounders, to estimate multivariate adjusted hazard ratios (HRs) with 95% confidence intervals (CIs). Never smokers, excluding passive, served as the reference group. We estimated attributable fractions in smokers and in the population with 95% CIs.
Results: Ever compared with passive and never smokers were younger at breast cancer diagnosis, at first childbirth and at menopause; they were less likely to have higher education, more likely to have used hormonal contraceptives and postmenopausal hormone therapy and to consume alcohol. The alcohol drinkers were consuming more alcohol. During follow-up 4293 women developed invasive breast cancer confirmed by histology. Compared with never smokers, the multivariate adjusted breast cancer HR was for ever smokers 1.21 (1.08-1.34). Compared with parous never smokers, the HR estimate for breast cancer for ever smokers who had smoked five or more years before giving birth was 1.29 (1.14-1.46) after adjustment. A trend test for number of pack-years and breast cancer risk was significant (ptrend= 0.007). We found similar HR estimates when we stratified by menopausal and parous status at entry. The attributable fraction for breast cancer was 17.3 (7.4-25.4) for active smokers. The population attributable fraction of breast cancer for active smoking was 11.9 (5.3-18.1).
Conclusion: In smokers, one in six, and in the population almost one in eight breast cancer cases could have been avoided in the absence of smoking.
Citation Format: Gram IT, Little MA, Lund E, Braaten T. The smoking related risk of breast cancer and proportion of avoidable breast cancer cases due to passive and active smoking in middle-aged women in Norway in 2012: The Norwegian women and cancer study 1991-2012. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-07-20.