Friday, February 6, 2015

Current Evidence on Smoking and Breast Cancer

Photo: flickr/saneboy (Creative Commons lic; cropped)
Though the most recent Surgeon General’s report on the health effects of tobacco stops just short of classifying smoking as a cause of breast cancer, current evidence seems compelling enough to finally push tobacco smoke from simple risk factor to full-fledged “cause.”

While research into the tobacco/breast cancer link can be confounded by certain differences in the habits of smokers and non-smokers - such as cancer screening frequency and alcohol consumption - studies that take such factors into account have found a fairly consistent impact of smoking on breast cancer risk.

Meta-analyses from the Surgeon General’s report, which combined the results from numerous case-control and cohort studies, are remarkably similar in their findings, showing around a 10 – 15 percent increase in risk in women who actively smoke (or smoked) compared to non smokers (1).

Looking at more recent analyses, the large Cancer Prevention Study II, which followed over 73,000 women for up to 14 years, found that current smokers had a 25 percent higher risk of breast cancer compared to never smokers, while past smokers had a 10 percent higher risk (2). A related meta-analysis, which included data from 14 others studies, found that both current and former smokers had a 10 percent higher breast cancer risk compared to never smokers.

These broad numbers, though, likely don’t paint the full picture of the tobacco/breast cancer link. As with other risk factors, such as alcohol consumption, studies are showing that smoking early in life – between the first period (menarche) and first full-term pregnancy – may have the greatest impact on cancer risk. A recently released long-term Norwegian study of over 300,000 women found that those women who smoked for at least 10 years before their first pregnancy had a 60 percent increased risk of breast cancer compared to those who never smoked (3). Results from the Cancer Prevention Study II found similar risk with smoking starting early in life.

While smoking doesn’t increase breast cancer risk on the order that it increases the risk of lung cancer, it nevertheless has a real and measurable impact. Healthy behaviors can prevent 50 percent or more of all breast cancers. On top of its many other health benefits, avoiding smoking - or stopping smoking -  should be included as a key way for women to take control and help lower their breast cancer risk.

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Evidence Summaries from 2014’s 
Health Consequences of Smoking—50 Years of Progress: A Report of the Surgeon General (1)

Smoking and Breast Cancer Incidence (New Cases)
“Based on 22 cohort reports published prior to 2012 and 27 case-control reports published from 2000– 2011, evidence suggests that a history of ever smoking is associated with an increase in the RR for breast cancer by an average of 10%; long duration of smoking (20 or more years), greater number of cigarettes smoked per day (20 or more), and more pack-years of smoking (20 or more) significantly increase risk for breast cancer by 13–16%, depending on study design and the exclusion of studies with design or analysis issues.”

Smoking and Breast Cancer Mortality
“To date, the evidence is insufficient to conclude that either active or passive smoking influences breast cancer mortality. Studies have been complicated by prob­lems with misclassifying exposure and a lack of specific­ity because smoking increases risk for several noncancer, comorbid conditions that contribute to mortality in survi­vors of breast cancer.”
Potential Causal Mechanisms
"The available evidence supports biologically plausible mechanisms, particularly for DNA adduct formation and unrepaired DNA mutations, by which exposure to tobacco smoke could cause breast cancer. However, data are limited and a detailed mechanistic model of how exposure to tobacco smoke may affect risk for breast cancer cannot yet be assembled. "


References
  1. U.S. Department of Health and Human Services. The Health Consequences of Smoking—50 Years of Progress: A Report of the Surgeon General. In: U.S. Department of Health and Human Services CfDCaP, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, ed. Atlanta, GA2014.
  2. Gaudet MM, Gapstur SM, Sun J, Diver WR, Hannan LM, Thun MJ. Active smoking and breast cancer risk: original cohort data and meta-analysis. J Natl Cancer Inst 2013;105:515-25.
  3. Bjerkaas E, Parajuli R, Weiderpass E, et al. Smoking duration before first childbirth: an emerging risk factor for breast cancer? Results from 302,865 Norwegian women. Cancer Causes Control 2013;24:1347-56.


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