Women who smoke for 35 years or more have a 59% higher risk of developing breast cancer, compared with those who never smoked, while those who smoked for 15-35 years had a 34% higher risk, according to data from the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial, a 5-year randomized placebo-controlled trial of tamoxifen in 13,388 healthy women at high risk of breast cancer because of family history or other factors. The data also show that smoking cigarettes is especially dangerous for this group of women.
The study, led by Stephanie Land, Ph.D., of the University of Pittsburgh, also looked at the incidence of invasive endometrial, lung, and colon cancers among its subjects, who were followed a median of 8.7 years. The investigators also looked at the effect of self-reported alcohol use and exercise habits on the risk of all four types of cancer.
In an abstract released in advance of the American Society of Clinical Oncology’s annual meeting in Chicago, Dr. Land and her colleagues reported that colon cancer incidence was also four times higher among women who reported having smoked more than 35 years than for never smokers, and 7% higher for women who smoked for 15-35 years.
"An increase in breast cancer risk associated with cigarette smoking had not been established until recently," Dr. Land said at a May 18 press conference announcing the findings and noted that her group’s study reported larger effects than had previously been seen.
While the findings show that smoking is even more dangerous for women at known risk of breast cancer, the good news is that "healthy lifestyle choices provide women a way to reduce their risk of these four major cancers," Dr. Land said.
Not surprisingly, longtime smokers saw a significantly higher risk of lung cancer in the investigators’ study. Women who smoked a pack of cigarettes per day for more than 35 years had a risk 30 times higher than did women who never smoked. Women who smoked less than one pack per day for more than 35 years had a 13-fold increase in lung cancer risk.
The investigators also found a significant association between low levels of physical activity and a 72% increased risk of endometrial cancer, which they hypothesized might be related to obesity, a known risk factor for endometrial cancer.
Alcohol consumption, however, was not associated with increased cancer risk, a finding that differs from previous studies. Moderate alcohol consumption of up to one drink a day was associated with a 65% decreased risk of colon cancer, compared with those women who did not drink. More than one drink per day was not associated with increased risk.
The investigators said several factors might have been different in this study from past studies, particularly that it enrolled fewer heavy drinkers (13.3% of subjects reported that they drank one or more drinks per day), compared with other studies.
In a press conference announcing these and other findings, Dr. George W. Sledge Jr., ASCO’s president and the Ballve-Lantero Professor of Oncology at Indiana University, Indianapolis, said Dr. Land’s study highlighted "the incredible importance of lifestyle factors," and offered a reminder of the need to "think less about drugs and a great deal about whether we can prevent cancer."
The findings, he said, offer "yet another reason to encourage women to stop smoking." Dr. Sledge said he also found the reduced risk of colorectal cancer associated with moderate drinking "very interesting," but that more research would be needed.
The study by Dr. Land and her colleagues was funded by the National Cancer Institute. One of Dr. Land’s coauthors, Dr. Donald Lawrence Wickerham, disclosed having consultant or advisory roles with Lilly and honoraria from AstraZeneca.