Background: Today, the majority of middle-aged women in Norway are ever (i.e. either former or current) smokers, who started to smoke in their teens. Many of those who never have smoked themselves were exposed to environmental tobacco smoke (ETS) as children. The purpose of the study, was to examine the effect of ETS in childhood on breast cancer risk in the Norwegian Women and Cancer Study, a nationally representative prospective cohort study.

Material and Methods: We followed 121 662 women, that were aged 34-70 years at enrolment, who completed a baseline questionnaire between 1991 and 2007, through linkages to national registries through December 2014. Questionnaire data included information on lifestyle factors, including lifetime history of smoking. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) while adjusting for relevant confounders and stratified by birth cohort. We included the following variables in the final multivariate models; age at enrolment, years of education (lt 10, 10-12, 13-16, 17+), age at menarche (lt 13,13-14, 15+), ever oral contraceptives use (yes, no), a variable including nulliparous and a combination of total number of births, (1, 2, 3+) and of age at first childbirth (age lt 20, 20-24, 25-29, 30+) for a total of 13 categories, family history of breast cancer in the mother (yes, no), body mass index (calculated from current height and weight (lt 20, 20-24.9, 25-29.9, 30+) kg/m 2, , menopausal status (yes, no), postmenopausal hormone therapy use (never, former, current) and average alcohol consumption, based on the content of pure alcohol in different sorts of beverages and portion sizes, as grams of alcohol per day (0, 0.1-1.40, 4.1-10, >10)). Women who reported to be teetotalers and those answering ‘seldom or never’ had their alcohol consumption set to 0.

Results: During a mean follow-up of 15 years, 4501 women developed invasive breast cancer confirmed by histology. Altogether, 23.7% of the women reported to be exposed to ETS during childhood. The women reported that the active smoker in 7.7% was the father, in 0.66 % the mother, in 3.57% both parents, and in 11.8 % the information about the active smoker was missing. Compared with never smokers, the multivariable adjusted HR estimate for ETS during childhood where the father was the smoker was statistically significantly increased with 23% (HR=1.23; 95% CI 1.07-1.42). The corresponding figures for those being exposed to ETS by the mother was a HR of 1.22 (95% CI 0.88-1.69), for those being exposed by both parents was a HR of 1.06 (95% CI 0.88-1.26) and for those with no information about the active smoker the HR was 1.03 (95% CI 0.89-1.20). In conclusion, we found that exposure to environmental tobacco smoke during childhood was associated with a higher risk of breast cancer for women who reported that the father was the active smoker.

Citation Format: Inger T. Gram, Tonje Braaten, Eiliv Lund, Idlir Licaj. Environmental tobacco smoke exposure and breast cancer risk in the Norwegian women and cancer study cohort [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2281. doi:10.1158/1538-7445.AM2017-2281