Smoking – especially early in life – linked to breast cancer risk
Where there’s smoke, there’s more likely to be breast cancer.
That’s the finding in a new study suggesting that women who begin smoking early in life – especially before the birth of their first child – appear to have an increased risk of breast cancer. The study was published Feb. 28 in the Journal of the National Cancer Institute.
Experts have long questioned whether smoking was linked to breast cancer, but studies have produced frustratingly contradictory findings. In 2004, for example, the surgeon general concluded there was no correlation between smoking and breast cancer. Over the years, after an estimated 130 epiemiologic studies and seven consensus reports examining the link between cigarette smoking and breast cancer risk, there is still no consensus.
In some cases, studies were complicated by the fact that women who smoke also drank alcohol, which has been linked to breast cancer. This study, led by Mia Gaudet, Ph.D., director of genetic epidemiology at the American Cancer Society, tried to tease out smoking as a potentially independent cause of breast cancer.
“The most consistent evidence we found to support a causal relationship between cigarette smoking and breast cancer risk was the link identified for women who start smoking before having their first child,” said Gaudet in a news release. “The relationship with early life smoking that we and others have found, together with the lack of a consistent relationship between breast cancer risk and smoking later in life, suggests that active cigarette smoking may play a greater role in the initiation than the progression of breast cancer.”
Gaudet and her colleagues studied the cases of 73,388 women enrolled in the American Cancer Society’s Cancer Prevention Study II (CPS II) Nutrition Cohort, a prospective study of predominantly postmenopausal women.
Researchers found that the incidence of invasive breast cancer was higher in current or former smokers than in those who had never smoked. Studying the records of women over a period of nearly 14 years, they identified 3,721 cases of invasive breast cancer.
They found incidence of invasive breast cancer was 24 percent higher in current smokers and 13 percent higher in former smokers, compared to those who never smoked.
"This paper is another important step toward the conclusion that smoking is a risk factor [for breast cancer] on its own," said Lacey, who was not involved in this research.
“Previous studies have explored whether smokers have a higher risk of breast cancer but this analysis focused on whether smoking at younger ages might be particularly associated with breast cancer,” said Lacey.
Meta-analyses are often criticized – sometimes unfairly – because they are secondary analyses of existing data, Lacey said. “But this is an example of where a good meta-analysis can be extremely valuable for understanding what we call modest risk factors – those that increase risk by a little bit, rather than a lot. Smoking appears to be one of those modest risk factors for breast cancer, and this paper makes a good argument for that point."
For Lacey, the study’s most surprising finding was that there was no "dose response" (increased risk due to a longer duration or intensity of smoking).
“Normally we would expect women who smoked more cigarettes or smoked for more years to be at the highest increased risk, but that's not the case in these recent studies. Seeing a dose response is reassuring from a biologic standpoint, so I think there is still some work to be done to understand precisely how smoking might affect breast cancer risk.”
Lacey was intrigued in the study’s finding that the window before a woman has children seems to be the period of most concentrated increased risk.
“It should allow our lab colleagues to look more closely at this window," Lacey said. “Among the questions to be answered is this: ‘Is the smoking making the tissue more susceptible to other cancer-causing agents or is it starting the cancer in the breast?’”
Gaudet said experts believe breast tissue is more susceptible to genotoxic exposures before women give birth.
“Mammary tissue is thought to be more susceptible to genotoxic exposures before completion of the first full-term pregnancy because the terminal ductal-lobular units of the breast are not fully differentiated until the end of gestation,” she wrote in the study.
Added Lacey: “There are so many reasons for all of us, but especially young people, to avoid tobacco smoke."
For young women inclined to light up, said Lacey, this study should give them pause.
“If it turns out that smoking during those years could have long-lasting effects on breast tissue in ways that increase a woman's risk of breast cancer years later,” he said, “then this adds one more reason for young women to never pick up that first cigarette or to make that next cigarette their last.”
Said Gaudet in the Healthday article: “Women who started smoking before their first menstrual period were 61 percent more likely (to get breast cancer than nonsmokers),” Gaudet said. “Women who took up the smoking habit after their period had started but 11 or more years before giving birth were at a 45 percent higher risk, compared to non-smokers.”