Women who feel they need another good reason to quit smoking might want to consider their breast cancer risk.
Those afflicted with breast cancer who smoke or who have smoked in the past are much less likely to survive the disease, according to a new study by UCSF epidemiologist Dejana Braithwaite, PhD.
Braithwaite, presenting her preliminary results at the American Association for Cancer Research’s annual “Frontiers in Cancer Prevention” meeting in Philadelphia on Nov. 8, reported that smokers or former smokers diagnosed with breast cancer were about 40 percent more likely to die as a result of the disease than women who had never smoked.
The study included 2,265 woman diagnosed with breast cancer. After diagnosis women were surveyed about smoking and followed to keep track of their health status. The median time of follow-up was nine years. During follow-up 164 of the women died from breast cancer, while 120 died from other causes. The results were independent of age, education, tumor characteristics and post-operative treatment.
The association in the study between smoking and a greater likelihood of death due to breast cancer was even larger among women diagnosed with a type of cancer called HER2-negative breast cancer. The preliminary findings suggest that women with HER2-negative breast cancer who smoked at the time of diagnosis or at some point in time prior to diagnosis were roughly 60 percent more likely to die from the disease in comparison to women who never smoked.
In addition, smokers with breast cancer were twice as likely to die from causes other than breast cancer. “It is unclear,” Braithwaite said, “whether there is an association between smoking and life expectancy following breast cancer diagnosis that works through affecting non-breast cancer causes of death.”
Smoking Affects Breast Cancer through Unknown Biological Mechanisms
“This is one of the largest studies yet to look at smoking among women diagnosed with breast cancer, and among the first to show such a significant association,” Braithwaite said.
Previous studies might have been less likely to clearly identify an association because they were too small, or because researchers relied on past clinical records rather than following women in real time, according to Braithwaite.
“We do not fully understand the biological mechanisms through which smoking affects breast cancer outcome,” Braithwaite said. She noted that animal studies have shown that chemicals in tobacco smoke can get into breast tissue and that exposure to tobacco smoke increases metastasis -- the spread of cancer to other tissues, and an event that most often leads to cancer deaths.
In addition, Braithwaite said, studies on mice led by Stanton Glantz, PhD, a UCSF tobacco researcher, showed that exposure to tobacco smoke stimulated tumors to grow.
“Her study is very well conducted,” said Glantz, director of the Center for Tobacco Control Research and Education. “What our work is showing is that there is something in tobacco smoke -- probably not nicotine -- that facilitates tumor growth. It’s very angiogenic, and generating new blood vessels to feed tumors is very important to cancer.”
In their studies of second-hand smoke exposure, Glantz and colleagues found that drugs that block nicotine did not interfere with the angiogenic effects of tobacco smoke. On the other hand, statins reduced angiogenesis in the exposed mice, which suggested to Glantz that the angiogenic response to smoke probably was triggered by inflammation.
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UCSF News Release, September 24, 2010