MONDAY, Nov. 8 (HealthDay News) -- Breast cancer patients who smoke or previously smoked have a higher risk of dying than nonsmokers with breast cancer, new research finds.

"Women who were smokers or had a history of smoking had a 39 percent higher rate of death due to breast cancer," said study author Dr. Dejana Braithwaite, an assistant professor of cancer epidemiology at the Helen Diller Family Comprehensive Cancer Center of the University of California, San Francisco. Their risk of death from other causes was also elevated, she said.

The findings are to be presented at the American Association for Cancer Research's annual meeting on cancer prevention research in Philadelphia, which runs through Wednesday.

Previous studies looking at the relationship between active smoking (as opposed to secondhand smoke) and survival among women with breast cancer have produced mixed findings, she said.

"The strength of our study is, it's a very large study of over 2,000 women diagnosed with breast cancer" at two sites in the United States, she said.

For this study, Braithwaite followed 2,265 women of many ethnicities, all diagnosed with breast cancer between 1997 and 2000, for nine years on average. The researchers looked at whether smoking affects breast cancer-related death rates and death rates from other causes.

During the follow-up, 164 women died of breast cancer and another 120 from other causes.

Besides a 39 percent higher rate of dying from breast cancer, the smokers and former smokers had an even higher rate of dying from other causes -- twofold -- compared to never smokers. Past smoking was defined as having smoked 100 or more cigarettes in their lifetime.

Looking at subgroups, Braithwaite found that the women most affected by smoking were those with tumors known as HER2 negative, those of lower body weight, and those past menopause.

The link can't be explained definitively, she said. One possibility is that the chemicals in tobacco smoke can make breast cancer more aggressive. But the assumption is, the longer one has gone without smoking, the less the risk, Braithwaite said.

In the study, 893 were former smokers, 173 current and 1,199 never-smokers.

"Their study is important," said Daniel Wartenberg, a professor of epidemiology at the Environmental and Occupational Health Sciences Institute at the Robert Wood Johnson Medical School in Piscataway, N.J. He previously studied the effect of passive smoking on breast cancer death rates and found no link.

If these findings are confirmed by other studies, he said, "that's a really important message, that getting people to stop smoking and prevent exposure may have a significant effect on reducing cancer deaths."

More information

To learn more about risk factors for breast cancer, visit the American Cancer Society.