Smoking and Breast Cancer: Is there a Connection?
—Katherine McKenzie, Ph.D.
Is there a connection between smoking and breast cancer? That is the question that was addressed at a workshop sponsored by the California Breast Cancer Research Program, the Tobacco Related Diseases Research Program, and the American Cancer Society. On December 4, 2002, the three programs convened a panel of experts to discuss what we know and don't know about the connection between breast cancer and smoking.
The question seems easy enough—does smoking increase one's risk of developing breast cancer? It's a valid question because, as panelist Dr. Stephen Hecht of the University of Minnesota pointed out, there are over 60 known carcinogens in cigarettes, 10 of which cause breast cancer in rodents. Additionally, some of these compounds can be detected in the breasts of smokers. However, studies in humans have been performed and the answer appears to be in: if you look at the population as a whole, smoking does not cause breast cancer. In fact, panelist Dr. John Baron of Dartmouth University went so far as to say that the question has been answered.
Is this the end of the story? What about passive smoking? Does genetics play a role in determining whether smoking can lead to the development of breast cancer? Does the time of life or amount of smoking affect breast cancer? What about the time of life when you quit—does that make a difference? Does smoking have implications for breast cancer prognosis? If we are going to get to the bottom of this issue, we will need to ask smarter questions and develop better tools to answer them.
Dr. Marilee Gammon of the University of North Carolina described the epidemiological efforts to answer these questions and the difficulties encountered in designing the studies. For instance, some studies define intensity and duration of smoking differently or some simply compare “non-active smokers” to “active smokers,” which taints their results because they are grouping non-smokers with passive smokers. Additionally, the concurrent use of alcohol, which has also been linked to breast cancer risk, can confound analyses. Passive smoking exposure has historically been difficult to quantify and is increasingly correlated with other aspects of lifestyle, such as alcohol consumption. These types of confounding factors make it hard to determine the precise role of smoking in breast cancer. However, there are tantalizing observations that certain segments of the population may be more susceptible to toxins in smoke because of their ability (or inability) to activate or detoxify them. Dr. Gammon argued that getting better answers to these questions will require multidisciplinary studies with sample sizes large enough to be able to understand the possible gene environment interactions involved.
Dr. Susan Murin of the University of California, Davis, discussed a relatively unexplored line of inquiry—how does smoking affect breast cancer prognosis? The data on this question is extremely scant, but several studies have shown that breast cancer patients who are smokers at the time of their diagnosis have a significantly higher risk of developing lung metastasis and have a higher chance of dying from the disease. Dr. Murin is currently funded by CBCRP and TRDRP to investigate the mechanism behind this increased risk. The preliminary results from Dr. Murin's mouse study indicate that smoking cessation at the time of diagnosis is effective in changing the course of the disease.
So, does smoking cause breast cancer? While smoking may not increase breast cancer in the population as a whole, it may increase risk in susceptible individuals and there may be implications for breast cancer prognosis. Additional total population studies on active smoking may not be necessary, but refined studies on questions such as the influence of timing of smoke exposure and effect on breast cancer prognosis can still yield valuable answers.
