Smoking raises the risk of breast cancer modestly, according to a report in the Jan. 24 issue of Archives of Internal Medicine.
Several measures of smoking – including the current quantity of cigarettes smoked, the past quantity of cigarettes smoked, the age at smoking onset, the duration of smoking, and total pack-years of smoking – correlated with risk of breast cancer in an updated analysis of data from the Nurses’ Health Study (NHS).
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An updated analysis of data from more than 111,000 women showed that a greater risk of breast cancer was associated current and past quantity of cigarettes smoked, age at onset and duration.
The strongest association was found in women who began smoking before giving birth for the first time, said Dr. Fei Xue of Brigham and Women’s Hospital and Harvard Medical School, Boston, and associates.
There have been numerous previous studies of this issue, but they have yielded a mix of positive, inverse, and null associations. In what the investigators described as "the largest [study] so far on the association between smoking and breast cancer risk," Dr. Xue and colleagues assessed data on 111,140 women participating in the prospective NHS from 1976 through 2006.
During that time there were 8,772 incident cases of breast cancer.
Women who had ever smoked had a "marginally increased" incidence of the disease compared with those who had never smoked, and the rate appeared to be comparable between current smokers and past smokers. "Every increase of 20 pack-years of smoking after menarche was associated with a marginal increase of incidence of breast cancer after adjusting for other risk factors," the researchers said.
Breast cancer risk was elevated in women who smoked 25 or more cigarettes per day either currently or in the past, women who began smoking at or before the age of 17, and women who smoked for 20 years or longer. "In most of the previous studies, these smoking measures were not mutually adjusted. In [our] study, we created an index of active smoking that integrates quantity, age at which one started smoking, and duration of smoking.
"The results suggested that, although an elevated risk for light smokers and moderate smokers was not apparent, heavy smokers who started smoking early in life, smoked for a long duration, and smoked a high quantity were at the highest risk of breast cancer, supporting an independent and additive effect from various smoking measures on breast carcinogenesis," they said (Arch. Intern. Med. 2011:171:125-33).
Starting to smoke before the first birth also was strongly associated with breast cancer. "All previous studies that have separately evaluated smoking before and after the first birth have found a similar pattern, suggesting that smoking before the first birth may be more important to breast carcinogenesis than smoking after the first birth," Dr. Xue and associates wrote.
"Smoking before menopause was positively associated with breast cancer risk, and there were hints from our results that smoking after menopause might be associated with a slightly decreased breast cancer risk. This difference suggests an antiestrogenic effect of smoking among postmenopausal women that may further reduce their already low endogenous estrogen levels.
"Conversely, among premenopausal women, any antiestrogenic effect of smoking may not be strong enough to significantly reduce endogenous estrogen levels, leaving the dominant carcinogenic effect of smoking," the researchers said.
In contrast to active smoking, passive smoking in childhood or adulthood was not associated with breast cancer risk. "Our results combined with the evidence from previous prospective cohort studies collectively suggest that passive smoking may not play an important role in the etiology of breast cancer. Nonetheless, we found that regular exposure to passive smoking may magnify the effect of active smoking," Dr. Xue and colleagues wrote.
The Nurses’ Health Study was funded by the National Cancer Institute. Dr. Xue and associates reported no relevant financial disclosures.