The study, which has an average follow-up of 9 years, provides strong evidence that smoking increases a woman’s risk of dying from breast cancer. Previous evidence had not been crystal clear.
“The epidemiological evidence has not been consistent with regard to the role of smoking in breast cancer mortality,” study lead author Dejana Braithwaite, PhD, told Medscape Medical News. She is an assistant professor in the Department of Epidemiology and Biostatistics at the University of California, San Francisco.
Dr. Braithwaite presented the new study here at the Ninth Annual American Association for Cancer Research International Conference on Frontiers in Cancer Prevention Research.
The study also showed that current and former smokers had a 2-fold higher rate of dying from nonbreast-cancer causes than never smokers. In evaluating breast cancer mortality and all-cause mortality among the women, Dr. Braithwaite and colleagues factored in other clinical, sociodemographic, and lifestyle-related factors.
They found that the adverse effect of smoking on breast cancer survival was highest among those with HER2-negative tumors, a body mass index below 25 kg/m2, and a postmenopausal status.
“Bottom line, there is a need for continuing improvement in smoking cessation efforts at the clinical provider and community levels,” she said.
Smoking cessation is tough, even with the motivating factor of a breast cancer diagnosis, said another expert.
“I’m not sure whether patients are more likely to quit smoking with a breast cancer diagnosis,” said Quyen Chu, MD, director of surgical oncology at the Feist-Weiller Cancer Center in Shreveport, Louisiana. He did not attend the cancer prevention conference.
Even the potential for a failed reconstruction of the breast might not be sufficient motivation, Dr. Chu suggested.
“I’ve had patients who’ve undergone reconstruction and I advise them to stop smoking, otherwise the reconstructive flap will break down,” he said. “I can’t tell you the number of times that patients don’t adhere to this. They will come back with a failed reconstruction and the first thing that I ask them is whether they stopped smoking. The answer is almost always no.”
Best Study to Date
Previous studies on the subject of smoking and breast cancer mortality have been weakened, said Dr. Braithwaite, by the fact that they were small in size, retrospective, or did not differentiate by the type of breast cancer.
This study, which follows women from northern California and Utah, has the strengths of being a relatively large sample size, prospective, multiethnic, and having multiple prognostic factors measured, she said.
The 2265 women were all diagnosed with breast cancer between 1997 and 2000. About 80% of the women had early-stage breast cancer.
There were 164 deaths from breast cancer and 120 deaths from nonbreast-cancer causes during the median follow-up of 9 years, said Dr. Braithwaite.
As noted above, compared with never smokers, women who were current or past smokers had a 39% higher rate of dying from breast cancer (hazard ratio [HR],1.39; 95% confidence interval [CI], 1.05 - 1.84) and a 2-fold higher rate of dying from competing (nonbreast-cancer) causes (HR, 2.16; 95% CI, 1.57 - 3.00).
Subgroup analyses revealed that the adverse effect of smoking on breast cancer survival was highest among women with HER2-negative tumors (HR, 1.61; 95% CI, 1.12 - 2.32), a body mass index below 25 kg/m2 (HR, 1.83; 95% CI, 1.10 - 3.04), and postmenopausal status (HR, 1.47; 95% CI, 1.08 - 1.99).
Although it is now certain that smoking increases a woman’s risk for death from breast cancer, it is not clear what biologic mechanisms promote the pathology, said Dr. Braithwaite.
“Other studies [have found] that carcinogenic chemicals in cigarettes penetrate the breast tissue and have even been found in the breast milk,” she said.
She also said that cigarette smoke has been found to affect the metastatic potential of tumor cells and to stimulate angiogenesis.