A review of published studies suggests that both active and passive smoking may increase the risk of breast cancer-particularly premenopausal breast cancer. These results were published in the International Journal of Cancer.
Although both active and passive smoking (exposure to second-hand smoke) are known to increase the risk of lung cancer, findings for breast cancer have been mixed.
In order to summarize what is known about the link between smoking and breast cancer, a researcher pooled information from 19 published studies. The studies were published between 1966 and 2004; they included both case-control studies and cohort studies.
In a cohort study, subjects are enrolled and exposure information is collected before disease develops. Subjects are then followed to identify those who develop disease. A case-control study, in contrast, enrolls subjects with disease (cases) and without disease (controls) and collects information about prior exposures. Case-control studies are generally more prone to bias than cohort studies because subjects with disease may recall past exposures differently than subjects without disease. In addition, subject participation or selection may vary by both disease and exposure status in case-control studies, producing a biased sample of study subjects.
Because they are less prone to bias, results from cohort studies are generally more persuasive than results from case-control studies. Three large cohort studies from the U.S. have reported no association between passive smoking and breast cancer risk.
In addition to considering study design, the researcher who pooled the data also considered how complete exposure information was. A study was classified as having complete information about passive smoking if the study collected information about childhood exposure from parents, adult exposure in the home, and adult occupational exposure. Only five of the 19 studies were considered to have complete information about passive smoking, and all five of these were case-control studies.
The results of the pooled analysis suggest that both active and passive smoking may increase the risk of breast cancer, particularly premenopausal breast cancer. Evidence of a link was particularly strong in the five studies that had complete information about passive smoking.
The following observations are based on information from all 19 studies:
- Passive smoking increased the risk of breast cancer by 27%.
- Among premenopausal women, passive smoking increased risk of breast cancer by 68%.
- Active smoking increased the risk of breast cancer by 46%.
The five studies with complete information about passive smoking produced the following information:
- Passive smoking increased the risk of breast cancer by 90%.
- Among premenopausal women, passive smoking increased risk of breast cancer by more than two-fold.
- Active smoking increased the risk of breast cancer by more than two-fold.
The researchers suggest that the lack of a link between passive smoking and breast cancer risk that was reported by three large U.S. cohort studies may have resulted from incomplete information about lifetime exposure to second-hand smoke. The researchers conclude that “Studies with thorough passive smoking exposure assessment implicate passive and active smoking as risk factors for premenopausal breast cancer.” Cohort studies with complete information about exposure to second-hand smoke would help substantiate these findings.
Reference: Johnson KC. Accumulating Evidence on Passive and Active Smoking and Breast Cancer Risk. International Journal of Cancer. 2005;117:619-628.