Being a current smoker or having a history of smoking significantly increased the risk of breast cancer progression and overall death among a group of multiethnic women with breast cancer, according to the results of a large prospective cohort study.

"We found that women who are current smokers or have history of smoking had a 39 percent higher rate of dying from breast cancer, even after we took into account a wide array of known prognostic factors including clinical, socioeconomic and behavioral factors," said Dejana Braithwaite, Ph.D, assistant professor, division of cancer epidemiology, department of epidemiology and biostatistics at the University of California, San Francisco.

Researchers presented these results at the Ninth Annual AACR Frontiers in Cancer Prevention Research Conference, held Nov. 7-10, 2010.

Although smoking is associated with lung cancer and implicated in several other cancers, it is unclear what effect smoking has on breast cancer, according to Braithwaite.

"Specifically, it is unclear how long women live following breast cancer diagnosis and whether smoking increases the risk of death because of breast cancer progression or whether there is an association between smoking and life expectancy following breast cancer diagnosis that works through affecting non-breast cancer causes of death," she said.

Therefore, Braithwaite and colleagues set out to examine the relationship between smoking and the risk of death due to breast cancer progression or non-breast cancer causes of death in a large group of women.

They enrolled 2,265 multi-ethnic women diagnosed with breast cancer between 1997 and 2000. The women were followed for an average of nine years. Researchers examined whether smoking affected death from breast cancer, non-breast cancer related causes and death from all causes.

Results showed that 164 deaths from breast cancer and 120 deaths from non-breast cancer causes occurred during follow-up.

Those women who had a history of smoking or who were current smokers also had a twofold increased risk for dying from non-breast cancer related causes compared with women with breast cancer who had never smoked.

An analysis was also conducted to examine whether body mass index, molecular breast cancer subtype or menopausal status modified risk. Women who were current or past smokers and also had a HER2-negative tumor subtype had a 61 percent increased risk for breast cancer death compared with those who never smoked. Smokers with a body mass index less than 25 kg/m2 had an 83 percent increased risk for breast cancer death, and postmenopausal women had a 47 percent increased risk for breast cancer death compared with those who never smoked.

"The implication of this research is that it is important for physicians to improve smoking cessation efforts, especially among women newly diagnosed with breast cancer, in order to improve breast cancer specific outcomes and overall health outcomes," Braithwaite said.