Long-term Smoking Is Associated With Up To 40 Percent Increased Risk Of Breast Cancer

What'smore, the researchers found that long-term smokers who add combinationhormone-replacement therapy (estrogen plus progestin) to the mixincrease their odds of getting breast cancer by 110 percent: more thandouble that of women who've never smoked or taken HRT.

Thesefindings, by Christopher I. Li, M.D., Ph.D., and colleagues, appearonline and will be published in the October print edition of CancerCauses and Control.

While a number of studies have looked at theassociation between smoking and breast cancer, many have beeninconclusive and many have had conflicting results, Li said, largelybecause of limitations in data collection, such as not taking intoaccount the duration or intensity of smoking or the timing of smokingonset. In addition, few previous studies have focused on older,postmenopausal women who've had a particularly long smoking history.

"Oursis one of the only population-based studies of its kind to focus on theassociation between smoking and breast-cancer risk in older womenbetween the ages of 65 and 79. Those who did smoke had much longerhistories of smoking than women in previous studies, so we were able tolook at the effects of long smoking durations on breast-cancer risk,"said Li, an assistant member of the Hutchinson Center's Public HealthSciences Division.

This study is the first of its kind to examinea wide variety of smoking parameters, such as how long and how often awoman has smoked, the number of pack years smoked, whether she was aformer or recent smoker, her age at smoking onset, and whether shestarted smoking before her first full-term pregnancy.

"We found a30 to 40 percent increased risk of breast cancer among women who werecurrent or long-term smokers, women who started smoking at a youngerage and also women who started smoking before their first full-termbirth," Li said.

The timing of smoking onset, particularly inrelation to first pregnancy, may be related to breast-cancer riskbecause of the known protective effect of pregnancy on breast tissue."During pregnancy, breast cells undergo a process calleddifferentiation, which makes them less susceptible to carcinogens,whereas breast cells in women who have never given birth are lessdifferentiated and therefore may be more vulnerable to carcinogenicinsults from the toxins in cigarettes," Li said.

Thepopulation-based case-control study, funded by the National CancerInstitute, involved nearly 2,000 older women in the Seattle-Puget Soundmetropolitan area. Half of the women had a history of breast cancer andhalf served as a healthy control, or comparison, group. Studyparticipants were interviewed in person and were asked about a varietyof factors known to influence breast-cancer risk, including hormone andalcohol use, and reproductive and family history. Even afterstatistically controlling for these factors, smoking emerged as asignificant, independent risk factor for breast cancer, Li said.

Perhapsthe most surprising finding surrounded the observation that combinedhormone therapy appears to significantly increase breast-cancer risk inwomen who smoke long term (those with a history of 20 or more packyears of smoking).

"We are really not sure what that findingmeans, because this correlation hasn't been reported in prior studies,"Li said. "We only saw the association in smokers who used both estrogenand progestin and not among women who used estrogen alone. We willfollow up on this finding in future studies to see if it can bereplicated."

Another interesting – and encouraging – finding wasthat once a woman stops smoking, within about 10 years of quitting herrisk of breast cancer falls back to that of a never-smoker. Thissuggests that recency of smoking may be particularly important withrespect to breast-cancer risk, Li said.

"We know that smoking isassociated with a lot of diseases, from lung cancer to heart disease,but the association with breast cancer is still somewhatcontroversial," said Li, who is also an assistant professor ofepidemiology at the University of Washington School of Public Healthand Community Medicine. "Certainly, the association between smoking andbreast cancer is nowhere near as strong as the association betweensmoking and lung cancer, but breast cancer may be another disease toadd to the long list of serious health issues related to smoking."