Cigarette Smoking and the Risk of Breast Cancer
Study Design: Prospective cohort study of Nurses’ Health Study participants
Study Eligibility: Female nurses aged 30 to 55 and living in the 11 most populous states in the US in 1976. Women who did not have prevalent invasive or in situ breast cancer or other cancers and who provided information on smoking in 1976, the first year of the study, were included in the active smoking analysis. Inclusion criteria for the passive smoking analysis were the same, albeit for never smokers. Questions on passive smoking were not asked until 1982; therefore women who provided information on passive smoking that year were included.
Enrollment: 111,140 participants for the active smoking analysis and 36,017 for the passive smoking analysis
Research Question: Is there an association between active and passive smoking, as well as smoking during various reproductive periods, and breast cancer risk?
Many studies have explored the association between cigarette smoking and breast cancer risk, albeit with inconsistent results of positive associations, inverse associations, and even null association. Tobacco smoke contains potential breast carcinogens but also has been postulated to have an antiestrogenic effect, which may explain the research findings of an inverse association. A previous analysis of the Nurses’ Health Study suggested a slightly elevated risk of breast cancer among active but not passive smokers, and particularly among women who began smoking at a young age. In the present study, the researchers report on an updated analysis with many more years of follow-up which added 5,632 new cases of breast cancer.
The researchers examined the records of 111,140 women in the Nurses’ Health Study from 1976 to 2006 for active smoking and 36,017 women from 1982 to 2006 for passive (secondhand) smoking. Women were followed up biennially from cohort entry until diagnosis of invasive or in situ breast cancer, loss to follow-up, death, or the end of the study period in 2006.
From 1976 to 2006, a total of 8,772 incident cases of breast cancer arose during 3,005,863 person-years of follow-up. The risk of breast cancer was not much higher for ever smokers relative to never smokers (HR=1.06, 95% CI=1.01-1.10), and this increase seemed to be comparable for past smokers and current smokers. However, the risk was higher for premenopausal smoking, with a higher risk observed for every increase of 20 pack-years (HR=1.11, 95% CI=1.07-1.15), and this did not differ by hormone receptor status. Furthermore, women who smoked before first birth had an 18% (95% CI=1.10-1.27) increased risk of breast cancer for every increase of 20-pack years.
The researchers created an index for active smoking which integrates quantity, age at which the individual started smoking, and duration of smoking. Heavy smokers who started before age 18 and who smoked more than 25 cigarettes a day for more than 35 years had an increased risk of breast cancer compared to never smokers (HR=1.25, 95% CI=1.06-1.46). In summary, the development of breast cancer was moderately associated with a higher quantity of current and past smoking, younger age at smoking initiation, and smoking more cigarettes and for a longer period of time.
On the other hand, smoking after menopause was not associated with the incidence of breast cancer (HR=0.93, 95% CI=0.85-1.02), and this did not vary by use of hormone therapy. According to the authors, “there were hints from our results that smoking after menopause might be associated with a slightly decreased breast cancer risk.” However, the data suggest a null association between smoking after menopause and breast cancer risk; therefore suggesting an antiestrogenic effect of smoking based on these results is premature.
In the passive smoking analysis, frequency and duration of secondhand smoke in adulthood or childhood were not associated with an increase in breast cancer risk. However, the researchers had limited qualitative and updated assessment of passive smoking, which could result in the potential for misclassification and biasing of the results to the null. Due to the null and moderately positive associations observed in this analysis, the results merely add to the growing body of conflicting literature on the association between smoking and breast cancer. These results suggest that smoking at a young age, especially smoking before having your first child, may be associated with a slight increase in breast cancer risk. More studies are needed to confirm this observation. It is important to understand that smoking after menopause has not been shown to reduce or have no effect on the risk of breast cancer and carries other health risks.
Xue F, Willet WC, Rosner BA, et al. “Cigarette Smoking and the Incidence of Breast Cancer.” Arch Intern Med. 2011 Jan 24;171(2): 125-133.