What's more, the researchers found that long-term smokers who add combination hormone-replacement therapy (estrogen plus progestin) to the mix increase their odds of getting breast cancer by 110 percent: more than double that of women who've never smoked or taken HRT.
These findings, by Christopher I. Li, M.D., Ph.D., and colleagues, appear online and will be published in the October print edition of Cancer Causes and Control.
While a number of studies have looked at the association between smoking and breast cancer, many have been inconclusive and many have had conflicting results, Li said, largely because of limitations in data collection, such as not taking into account the duration or intensity of smoking or the timing of smoking onset. In addition, few previous studies have focused on older, postmenopausal women who've had a particularly long smoking history.
"Ours is one of the only population-based studies of its kind to focus on the association between smoking and breast-cancer risk in older women between the ages of 65 and 79. Those who did smoke had much longer histories of smoking than women in previous studies, so we were able to look at the effects of long smoking durations on breast-cancer risk," said Li, an assistant member of the Hutchinson Center's Public Health Sciences Division.
This study is the first of its kind to examine a wide variety of smoking parameters, such as how long and how often a woman has smoked, the number of pack years smoked, whether she was a former or recent smoker, her age at smoking onset, and whether she started smoking before her first full-term pregnancy.