While a number of studies in the past have looked at the association between smoking and breast cancer, many have been inconclusive and often had conflicting results. But a new study, in the journal Cancer Causes and Control, that takes into account the duration or intensity of smoking in postmenopausal women has uncovered some worrying links.
The study, from the Fred Hutchinson Cancer Research Center, involved nearly 2,000 older women. Half of the women had a history of breast cancer and half served as a healthy comparison group. After adjusting for factors such as alcohol use and family history, smoking emerged as a significant, independent risk factor for breast cancer, the researchers said.
It seems that 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 - 40 percent increased risk of developing breast cancer as compared to women who've never smoked. Even worse, long-term smokers who take combination hormone-replacement therapy (HRT) increase their odds of getting breast cancer by 110 percent, more than double that of women who've never smoked or taken HRT.
"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 researcher Christopher I. Li. "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."
But the most surprising finding was the observation that combined hormone therapy appears to significantly increase breast-cancer risk in women who smoke long term (those with a history of 20 or more pack-years of smoking). "We are really not sure what that finding means, because this correlation hasn't been reported in prior studies," Li said. "We only saw the association in smokers who used both estrogen and progestin and not among women who used estrogen alone. We will follow up on this finding in future studies to see if it can be replicated."
Encouragingly, once a woman stops smoking, within 10 years her risk of breast cancer falls back to that of a never-smoker. This suggests that currency of smoking may be particularly important with respect to breast-cancer risk. "Certainly, the association between smoking and breast cancer is nowhere near as strong as the association between smoking and lung cancer, but breast cancer may be another disease to add to the long list of serious health issues related to smoking," Li concluded.