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Does Tobacco Smoke Cause Breast Cancer?

ISSN 1745-5057 Women's Health (2011) 7(4), 405–408 10.2217/WHE.11.39 © 2011 Future Medicine Ltd The null findings reported by the collaborative group have been challenged more recently since the crude definition of active smoking used in that ana lysis did not consider duration of smoking or time periods of potential vulnerability, such as exposure during the period between menarche and first full-term pregnancy [2,7]. Indeed, there are now several examples in the literature of studies in which long-term cigarette smoking or years of smoking before first fullterm pregnancy was more strongly associated with risk than was ever-smoking [2,3]. As yet, no pooled ana lysis has tested these hypotheses in large studies of women who report no regular consumption of alcohol. Although the majority of epidemiological studies have demonstrated no clear association with risk [3], several investigators and consensus reviews have concluded that a causal association between active smoking and breast cancer cannot be ruled out, given recent updates from large prospective studies reporting: positive associations (overall 10–30% increased risk) in women who actively smoked large numbers of cigarettes over many years; increased risk in women who smoked prior to a first full-term birth; and evidence from studies suggesting that alcohol consumption may not explain positive associations with smoking, including recent analyses in nondrinkers [8,9]. There is also conflicting evidence regarding whether the association with active smoking may be confined to genetically susceptible subgroups, particularly women with the NAT2-slow or very slow acetylation g enotypes [10–12]. Several scientific consensus groups have evaluated this evidence repeatedly between 2004 and 2009 and have reached different conclusions, although the assessments are generally converging for active smoking (Table 1). Both the International Agency for Research on Cancer (IARC) and the US Surgeon General Report reviewed the evidence for active smoking in 2004 and concluded that that there was evidence suggesting a lack of carcinogenicity [13,14]. Several years later, in 2009, another review by IARC changed the designation More than 1 billion people currently smoke tobacco [1], and tobacco consumption is the leading cause of cancer worldwide. Even more people, especially women in developing countries, continue to be exposed to second-hand smoke. Consequently, efforts to determine whether tobacco smoke affects breast cancer risk, and if so, the nature and strength of the relationships with active and passive smoking, have important public health as well as scientific implications. Some researchers have posited that tobacco smoke increases breast cancer risk as much or more so among women who are exposed passively to tobacco smoke as in those who smoke actively. If this were true, then reductions in active and passive smoking would be among the most important interventions for the prevention of breast cancer, in addition to their numerous other health benefits. However, to date, no scientific consensus has been reached on these issues, despite more than 20 years of debate and over 150 e pidemiological studies [1–3]. The views of breast cancer epidemiologists on these issues have fluctuated over the past 20 years. In the early 1990s, Baron and colleagues [4] proposed that active smoking might protect against breast cancer through its presumed antiestrogenic effects, as is thought to be the case with endometrial cancer [5]. However, subsequent studies failed to demonstrate this inverse relationship, showing either no association or a small increase in risk associated with active smoking [3]. The idea that smoking might increase breast cancer risk was later tested in a pooled collaborative ana lysis of 53 epidemiological studies with 83,000 breast cancer cases [6]. No association was observed between breast cancer and ever having smoked actively in the subgroup of women who reported no alcohol consumption. Therefore it was concluded, that residual confounding by alcohol consumption, an established risk factor for breast cancer, likely introduces a spurious positive association between smoking and breast cancer [6]. Although the authors suggested restricting future analyses of this issue to lifelong nondrinkers, few studies have been sufficiently large for this purpose. Editorial