Doctors have long suspected a link between smoking and breast cancer risk, but research results have been mixed. The study reviewed here combined and analyzed the results of many earlier studies and found that both smoking and exposure to secondhand smoke do raise breast cancer risk. Looking at the results from many different studies all looking at the same issue is called a meta-analysis.
The study found that breast cancer risk is higher in:
Post-menopausal women exposed to secondhand smoke didn't have an increase breast cancer risk in this study.
This study doesn't say how smoking increases breast cancer risk. We do know that cigarette smoke contains compounds called aromatic amines. Certain aromatic amines are known to cause cancer.
The study also confirmed a link between a gene and how much smoking affects breast cancer risk. Smoking and secondhand smoke exposure raises breast cancer risk more in women who have a slow-acting form of the NAT2 gene. The slow-acting NAT2 gene slows the body's ability to get rid of aromatic amines. Breast cancer risk in all long-term smokers is about 20% higher than in non-smokers. This study found that breast cancer risk was 27% higher in women with the slow-acting NAT2 gene who smoked. Other research has found that risk is 35% to 50% higher in long-term smokers with the slow-acting NAT2 gene.
About 50% to 60% of white women and 35% to 40% of African American women have the slow-acting NAT2 gene. Some research has suggested that women who smoke and don't have the slow-acting NAT2 gene don't have a higher risk of breast cancer. Still, smoking can harm your heart and lungs affect your overall health. A routine test for the slow-acting NAT2 gene isn't available, so women usually don't know if they have it.
Smoking can harm your health, including your breast health, at any age. If you don't smoke, don't start. If you do smoke, find a program or system to help you quit. Quitting is tough, but it's definitely worth it. The American Lung Association offers a free online smoking cessation program. Local chapters of the American Cancer Society offer the Fresh Start program to help people quit smoking. You can also call the ACS "Quitline" at 1-800-ACS-2345 to get support and free advice on how to stop smoking from trained counselors.
LITTLE FALLS, N.J., April 24 (MedPage Today) -- Both active smoking and secondhand smoke are causally linked to breast cancer, an international panel of researchers has found.
In its review of several recent studies and meta-analyses, the panel, convened by the University of Toronto and chaired by the university's Neil Collishaw, M.D., found that associations between active smoking and both pre- and postmenopausal breast cancer are "consistent with causality."
The panel also found that exposure to secondhand smoke was causally linked to breast cancer in premenopausal women.
"Until recently, evidence about the link between breast cancer and tobacco smoke, although voluminous, was inconclusive," Dr. Collishaw said in a statement. "But the panel's careful analysis of all available evidence, particularly recent evidence, led us to conclude that there is persuasive evidence of risk."
Historically, the epidemiological evidence regarding breast cancer and smoking has been conflicting, but more recent studies have added to evidence that smoking increases breast cancer risk.
Those studies have suggested that women who started smoking at an early age had a 20% increase in breast cancer risk, the panelists said in their report.
Stronger evidence comes from three recent meta-analyses and a pooled analysis, which found increases of 35% to 50% in breast cancer risk for long-term smokers who have one of several N-acetyltransferase 2 (NAT2) slow-acetylation genotypes.
About half of North American women have the genotype.
The researchers said the most convincing review, which looked at 13 studies, found that among women with the NAT2 genotype, those who smoked had about a 27% increase in risk of breast cancer compared with never-smokers (95% CI 1.16 to 1.39).
The panel also found that secondhand smoke poses a risk, with the California Environmental Protection Agency and the U.S. Surgeon General publishing meta-analyses in 2006 suggesting a 60% to 70% increase in breast cancer risk among premenopausal non-smokers who had regular, long-term exposure to secondhand smoke.
The California EPA review concluded the relationship was "consistent with causality" and the Surgeon General review said it was "suggestive" of a causal relationship.
Another meta-analysis with "good measurement of lifetime exposure to active and secondhand smoke" found both conditions doubled the risk of premenopausal breast cancer, the researchers said.
However, they said the evidence is insufficient to "pass judgment on secondhand smoke and postmenopausal breast cancer," although the panel can't rule out a causal association.
The researchers called for further study of risk in relation to age at smoking initiation, smoking before pregnancy, and duration of high pack-years.
They also called for further research into tobacco risk and targeted genotypes, particularly NAT2 and the BRCA1 and 2 mutations.
"Young women in particular should understand that available evidence suggests that the relationship between breast cancer and both active smoking and secondhand smoke is consistent with causality," the researchers said. "The public health implications of these findings highlight the need for effective messaging."
The researchers reported no conflicts of interest.
Primary source: Ontario Tobacco Research Unit Source reference: Collishaw NE, et al "Canadian expert panel on tobacco smoke and breast cancer risk" 2009.
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