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For years, experts have questioned whether cigarette smoking is directly linked with breast cancer risk or whether the association is complicated by the fact that many women who smoke also drink alcohol, which has also been tied to breast cancer risk.
Studies have produced conflicting results. When the U.S. Surgeon General last reviewed the issue in 2004, the report concluded that there was no cause-and-effect relationship between smoking and breast cancer risk.
"It's not just a relationship between alcohol and breast cancer, but in fact smoking by itself is related to breast cancer," said Mia Gaudet, director of genetic epidemiology at the American Cancer Society. She led the study, which was published online Feb. 28 in the Journal of the National Cancer Institute.
The timing of the smoking appears to affect the degree of risk, she said. "It seems that women who start smoking before their first birth are at greatest risk of breast cancer," Gaudet said.
The researchers looked at data from women enrolled in a large, long-term cancer society study involving lifestyle factors and prevention. Over the follow-up of nearly 14 years, more than 3,700 cases of invasive breast cancer were found.
When the women entered the study in 1992, they were aged 50 to 74. They supplied information on smoking habits, past and present. At the start, about 8 percent smoked, about 36 percent had quit and about 56 percent never smoked.
The incidence of invasive breast cancer was 24 percent higher in current smokers and 13 percent higher in former smokers, compared to never smokers, the researchers found.
The researchers next focused on the timing of the smoking. "Women who started smoking before their first menstrual period were 61 percent more likely [to get breast cancer than nonsmokers]," Gaudet said. Women who took up the smoking habit after their period had started but 11 or more years before giving birth were at a 45 percent higher risk, compared to nonsmokers.
To analyze whether the cigarette smoking by itself -- not the combination of drinking and smoking -- affects breast cancer risk, Gaudet first looked at both smoking and drinking in the same model. ''It showed that the relationship still existed between smoking and breast cancer," she said.
Next, she looked separately at the groups of drinkers --never, former and current -- and analyzed their risk of breast cancer. For the never drinkers, smoking now or in the past was not linked with breast cancer risk. Ideally, she said, all three groups would have a similar risk to prove smoking by itself is a risk factor. However, she acknowledged, "We did not see that."
Gaudet said she is not sure why that was, but that there may be something about the combination of smoking and drinking that affects breast cancer risk. "Or the numbers [of women] may be too small to show an accurate result."
Even so, she said, the new research suggests that smoking by itself drives up breast cancer risk. The researchers found a link or association, but cannot prove cause and effect.
One cancer expert praised the study.
"This paper is another important step toward the conclusion that smoking is a risk factor [for breast cancer] on its own," said James Lacey, an associate professor of cancer etiology at the City of Hope Comprehensive Cancer Center in Duarte, Calif. He was not involved with the research.
One interesting finding, he said, is that the window before a woman has children seems to be the period of most concentrated increased risk. "It should allow our lab colleagues to look more closely at this window," Lacey said. Among the questions to be answered, he said, is this: "Is the smoking making the tissue more susceptible to other cancer-causing agents or is it starting the cancer in the breast?"
Gaudet said that experts think breast tissue is more susceptible to toxic exposures before a woman gives birth the first time compared to after.
The study finding, Gaudet said, ''provides additional motivation for young women who are thinking of starting to smoke not to." Those who smoked while young and gave up the habit still had a higher risk of breast cancer, she said, than never smokers.
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