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"The findings are important because smoking was not previously thought to increase the risk of breast cancer, but this study adds to the increasing evidence that it does," said lead researcher Dr. Karen Margolis, a senior clinical investigator at HealthPartners Research Foundation in Minneapolis.
"When you put together the body of work in the last few years, it calls for more studies," she said. "This study has answered that call."
"This certainly adds to the evidence that long-term smoking increases the risk for breast cancer," Gapstur said. "On the on the flip side, it appears that 20 years after stopping the risk goes down to that of an average individual. I think that's good news."
Many risk factors for breast cancer cannot be changed, such as age, genetics and family history of the disease, Margolis noted.
"Now smoking can be added to the list of things that can lower breast cancer risk that already include having children, breast-feeding, keeping alcohol consumption low, avoiding weight gain, being physically active and avoiding hormone therapy with estrogen plus progestin," she said.
The report is published in the March 1 online edition of the BMJ.
For the study, Margolis's group collected data on 79,990 women aged 50 to 79 who took part in the Women's Health Initiative study. Over 10 years of follow-up, 3,250 women developed breast cancer.
As part of the study, the women were asked if they smoked, had stopped smoking or had never smoked. The women were also asked about their exposure to secondhand smoke at home and at work.
The researchers found that women who smoked had a 16% increased risk of developing breast cancer. Among women who quit, the increased risk was 9%, they added.
The greatest risk was for women who had smoked for 50 years or longer, compared with women who never smoked, Margolis's team found. The risk was also high for women who started smoking when they were teenagers. Even after quitting, the risk continued for up to 20 years, the researchers noted.
"We also observed some evidence that extensive exposure to passive smoking may raise the risk of breast cancer," Margolis said.
Nonsmokers exposed to secondhand smoke for more than 10 years as children, more than 20 years as adults at home and more than 10 years at work had a 32% increased risk of developing breast cancer, the researchers found.
However, the link between breast cancer and secondhand smoke was seen in those exposed to the greatest amount of passive smoking and "therefore more research is needed to confirm these findings," the researchers noted.
Dr. Paolo Boffetta, deputy director of the Tisch Cancer Institute and Institute for Transitional Epidemiology at Mount Sinai School of Medicine in New York City and co-author of an accompanying journal editorial, said that "tobacco smoking, particularly when started early in life, may increase the risk of breast cancer."
"This evidence is becoming stronger and stronger," he said. "In previous studies, the evidence was not so strong. It is only now that women who started smoking in large numbers are getting to the age where the risk of breast cancer is getting high."
Right now, the association between smoking and breast cancer is still not a sure thing, Boffetta said, "but, it is getting more likely."
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