June 24, 2011 -- Women who are struggling to exercise regularly, maintain a healthy weight, or turn down a nightly glass of wine just got a bit of breathing room, at least as far as breast cancer is concerned.
A new study shows that these modifiable risk factors, even when taken together, account for a relatively small portion of a woman's overall breast cancer risk.
"I was surprised, a little bit, that you really do get a really different view of what is achievable by looking at absolute risks as opposed to relative risks, or attributable risks. The absolute risk reductions are kind of small it seems to me," says study researcher Mitchell H. Gail, MD, PhD, a senior investigator in the division of cancer epidemiology and genetics at the National Cancer Institute in Bethesda, Md.
Gail is no lightweight in the field of risk prediction. His research led to the development of a widely used tool that calculates a woman's five-year risk of breast cancer based on things that can't be controlled, like age, family history, ethnicity, how old she was when she had her first child, and her age at her first period. The tool calculates a number that's known as the "Gail score."
His new study, which is published in the Journal of the National Cancer Institute, shows that non-modifiable risk factors may be more important to determining overall breast cancer risk than things that are controllable, like body mass index (BMI), leisure time physical activity, or alcohol consumption.
Analyzing Cancer Risk
To look at the risks, the study compared 2,569 breast cancer patients to 2,588 women of the same ages who did not have cancer. The women interviewed in detail about a host of things that could contribute to cancer risk like their socioeconomic status, education, occupation, smoking habits, physical activity, alcohol and coffee consumption, dietary habits, and their weights at various ages.
Based on their answers, researchers developed models to try to tease out the relative contributions of physical activity, body weight, and consumption of alcohol to their risk of breast cancer.
Here's an example. The study predicts that a woman who is 65 years old with no other risk factors will have an average risk of developing breast cancer within the next 20 years of about 6.5%. If that woman stopped drinking alcohol, started exercising for at least two hours each week, and maintained a normal body weight, her absolute risk of getting breast cancer would drop about 1.6%, to 4.9%.
Here's the part that often ends up in news headlines. Exercising, staying slim, and not drinking alcohol accounted for about 24% of her overall risk of getting breast cancer. So by doing all those things, she's reducing her risk for breast cancer by 24%, which sounds like a sizeable chunk. But it's only a part of an already small number.
"The really key factors for developing breast cancer are family history and increasing age. All of the other factors are relatively weak," says Ruth O' Regan, MD, an associate professor at Emory University's Winship Cancer Institute in Atlanta.
"These modifiable factors kind of fit into those slightly weaker factors," she says. "If you're at average risk, this is sort of an encouraging paper."
Experts are quick to add that beyond breast cancer risk, there are plenty of other reasons -- heart disease and diabetes among them -- to stay slim, get regular exercise, and have a moderate alcohol intake.
And they point out that lifestyle factors do make more of an impact, however, when a woman has more things in her breast cancer risk profile that she can't change.
According to the study, a 65-year-old woman with a family history of breast cancer has a 13.8% chance of developing breast cancer within the next 20 years. If she maintains a normal body weight (BMI under 25), exercises for at least two hours each week, and stops drinking alcohol, her absolute risk drops by 3.2%, to 10.6%, or a 23% drop in relative risk.
A 65-year-old woman who has the highest numbers of non-modifiable risk factors has a 20-year absolute risk of getting breast cancer of 18.6%. By exercising, watching her weight, and not drinking alcohol, she cuts that risk by 4.1%, for a 22% decrease in her relative risk, the study found.
Perspective on Cancer Risk
Experts praised the paper for adding some needed perspective to breast cancer risk prediction.
"Counseling for personal risk of breast cancer is really difficult, and I think this was a really, really well done paper," says Jennifer Litton, MD, assistant professor and a breast medical oncologist at the University of Texas, M.D. Anderson Cancer Center in Houston.
Litton cautions that the study was conducted in Italian women, who have slightly different risks and rates of breast cancer compared to Americans. "It's in a different population, and different populations and cultures have different risk factors and chances of developing cancer, so as far as how transferable it is to the United States will need to be seen," she says.
"I think this may provide us a very interesting tool, another aid when we're counseling patients, because there's so much we don't have control over," Litton says, "For the things we can change, I think it's helpful to understand."
Other experts, and the study researchers, point out that beyond individual risk, these kinds of models can also show the impact that relatively small personal changes can make to larger populations.
For example, the study found that in a population of 1 million women, a 1.6% reduction in absolute risk would translate to 16,000 fewer cancer cases.
"These kinds of models are more important at the population level than at the personal level," says Mary B. Daly, MD, PhD, chair of the department of clinical genetics at Fox Chase Cancer Center in Philadelphia.
"I've been taking care of women with breast cancer for 25 years, most of them don't have any of these risk factors," she says. "There's obviously a lot more going on. Those risk factors that we know about only tell part of the picture."