Older women who have smoked for 11 or more "pack years" - the lifetime equivalent of a pack a day for at least 11 years - face a 30 percent to 40 percent increased risk of developing breast cancer as compared to women who've never smoked, according to new findings from researchers at Fred Hutchinson Cancer Research Center.
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.
"We found a 30 to 40 percent increased risk of breast cancer among women who were current or long-term smokers, women who started smoking at a younger age and also women who started smoking before their first full-term birth," Li said.
The timing of smoking onset, particularly in relation to first pregnancy, may be related to breast-cancer risk because of the known protective effect of pregnancy on breast tissue. "During pregnancy, breast cells undergo a process called differentiation, which makes them less susceptible to carcinogens, whereas breast cells in women who have never given birth are less differentiated and therefore may be more vulnerable to carcinogenic insults from the toxins in cigarettes," Li said.