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      The fraction of breast cancer attributable to smoking: The Norwegian women and cancer study 1991–2012


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          Results from several recent cohort studies on smoking and breast cancer incidence and mortality suggest that the burden of smoking on society is underestimated. We estimated the fraction of breast cancer attributable to smoking in the Norwegian Women and Cancer Study, a nationally representative prospective cohort study.


          We followed 130 053 women, aged 34–70 years, who completed a baseline questionnaire between 1991 and 2007, through linkages to national registries through December 2012. We used Cox proportional hazards models to estimate hazard ratios (HRs) with 95% confidence intervals (CIs), while adjusting for confounders. Never smokers, excluding passive smokers, were used as the reference group in all main analyses. We estimated attributable fractions (AFs) % in smokers and in the population (PAFs) % with 95% CIs.


          Altogether, 4293 women developed invasive breast cancer, confirmed by histology. Compared with never active, never passive smokers, ever (former and current) smokers had an overall risk of breast cancer that was 21% higher (HR=1.21; 95% CI=1.08–1.34). For ever smokers, the AF was 17.3% (95% CI =7.4–25.4) and for the population the PAF of breast cancer was 11.9% (95% CI=5.3–18.1). For passive smokers, the PAF of breast cancer was 3.2% (95% CI=1.0–5.4). When we applied PAF estimates for ever smoking on the 2907 new breast cancer cases among Norwegian women aged 35+ at diagnosis in 2012, this yielded 345 (95% CI=154–526) breast cancer cases that could have been avoided in the absence of active smoking that year.


          In smokers, one in six and in the population, one in nine breast cancer cases could have been avoided in the absence of active smoking. Our findings support the notion that the global cancer burden due to smoking is substantially underestimated.

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          Mortality from tobacco in developed countries: indirect estimation from national vital statistics.

          Prolonged cigarette smoking causes even more deaths from other diseases than from lung cancer. In developed countries, the absolute age-sex-specific lung cancer rates can be used to indicate the approximate proportions due to tobacco of deaths not only from lung cancer itself but also, indirectly, from vascular disease and from various other categories of disease. Even in the absence of direct information on smoking histories, therefore, national mortality from tobacco can be estimated approximately just from the disease mortality statistics that are available from all major developed countries for about 1985 (and for 1975 and so, by extrapolation, for 1995). The relation between the absolute excess of lung cancer and the proportional excess of other diseases can only be approximate, and so as not to overestimate the effects of tobacco it has been taken to be only half that suggested by a recent large prospective study of smoking and death among one million Americans. Application of such methods indicates that, in developed countries alone, annual deaths from smoking number about 0.9 million in 1965, 1.3 million in 1975, 1.7 million in 1985, and 2.1 million in 1995 (and hence about 21 million in the decade 1990-99: 5-6 million European Community, 5-6 million USA, 5 million former USSR, 3 million Eastern and other Europe, and 2 million elsewhere, [ie, Australia, Canada, Japan, and New Zealand]). More than half these deaths will be at 35-69 years of age: during the 1990s tobacco will in developed countries cause about 30% of all deaths at 35-69 (making it the largest single cause of premature death) plus about 14% of all at older ages. Those killed at older ages are on average already almost 80 years old, however, and might have died soon anyway, but those killed by tobacco at 35-69 lose an average of about 23 years of life. At present just under 20% of all deaths in developed countries are attributed to tobacco, but this percentage is still rising, suggesting that on current smoking patterns just over 20% of those now living in developed countries will eventually be killed by tobacco (ie, about a quarter of a billion, out of a current total population of just under one and a quarter billion).
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            Active smoking and breast cancer risk: original cohort data and meta-analysis.

            The relationship between active cigarette smoking and breast cancer risk remains controversial because of unresolved issues of confounding and dose response. To investigate these issues further, we analyzed data from 73 388 women in the American Cancer Society's Cancer Prevention Study II (CPS-II) Nutrition Cohort. Analyses were based on 3721 invasive breast cancer case patients identified during a median follow-up of 13.8 years. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated from multivariable-adjusted Cox proportional hazard regression models. P values were two-sided. We also conducted meta-analyses of our results with those published from 14 other cohort studies. In CPS-II, incidence was higher in current (HR = 1.24, 95% CI = 1.07 to 1.42) and former smokers (HR =1.13, 95% CI = 1.06 to 1.21) than in never smokers. Women who initiated smoking before menarche (HR = 1.61, 95% CI = 1.10 to 2.34) or after menarche but 11 or more years before first birth (HR = 1.45, 95% CI = 1.21 to 1.74) had higher risk (P trend = .03). No relationships were observed with other smoking parameters. Alcohol consumption did not confound associations with smoking status, although neither current nor former smoking were associated with risk among never drinkers (P interaction = .11). In meta-analyses, current (HR = 1.12, 95% CI = 1.08 to 1.16) and former smoking (HR = 1.09, 95% CI = 1.04 to 1.15) were weakly associated with risk; a stronger association (HR = 1.21, 95% CI = 1.14 to 1.28) was observed in women who initiated smoking before first birth. These results support the hypothesis that active smoking is associated with increased breast cancer risk for women who initiate smoking before first birth and suggest that smoking might play a role in breast cancer initiation.
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              Active and passive smoking and risk of breast cancer: a meta-analysis.

              Studies on active and passive tobacco smoking and breast cancer have found inconsistent results. A meta-analysis of observational studies on tobacco smoking and breast cancer occurrence was conducted based on systematic searches for studies with retrospective (case-control) and prospective (cohort) designs. Eligible studies were identified, and relative risk measurements were extracted for active and passive tobacco exposures. Random-effects meta-analyses were used to compute summary relative risks (SRR). Heterogeneity of results between studies was evaluated using the (I (2)) statistics. For ever active smoking, in 27 prospective studies, the SRR for breast cancer was 1.10 (95 % CI [1.09-1.12]) with no heterogeneity (I (2) = 0 %). In 44 retrospective studies, the SRR was 1.08 (95 % CI [1.02-1.14]) with high heterogeneity (I (2) = 59 %). SRRs for current active smoking were 1.13 (95 % CI [1.09-1.17]) in 27 prospective studies and 1.08 (95 % CI [0.97-1.20]) in 22 retrospective studies. The results were stable across different subgroup analyses, notably pre/post-menopause, alcohol consumption adjustments, including/excluding passive smokers from the referent group. For ever passive smoking, in 11 prospective studies, the SRR for breast cancer was 1.07 (95 % CI [1.02-1.13]) with no heterogeneity (I (2) = 1 %). In 20 retrospective studies, the SRR was 1.30 (95 % CI [1.10-1.54]) with high heterogeneity (I (2) = 74 %). Too few prospective studies were available for meaningful subgroup analyses. There is consistent evidence for a moderate increase in the risk of breast cancer in women who smoke tobacco. The evidence for a moderate increase in risk with passive smoking is more substantial than a few years ago.

                Author and article information

                Br J Cancer
                Br. J. Cancer
                British Journal of Cancer
                Nature Publishing Group
                23 August 2016
                09 June 2016
                23 August 2016
                : 115
                : 5
                : 616-623
                [1 ]Faculty of Health Sciences, Department of Community Medicine, University of Tromsø (UiT) The Arctic University of Norway , Hansine Hansens veg 18, 9019 Tromsø, Norway
                [2 ]Center for Population Sciences, Department of Preventive Medicine, University of Tennessee Health Science Center , 66 N. Pauline Street, Memphis, TN 38163, USA
                Author notes
                Copyright © 2016 Cancer Research UK

                This work is licensed under the Creative Commons Attribution-Non-Commercial-Share Alike 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/


                Oncology & Radiotherapy
                ever and passive smoking,avoidable breast cancer,breast cancer risk,cohort study,norway,norwegian women and cancer study,population attributable fraction,smoking duration before first childbirth


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