Many women think that breast cancer runs in families, or that it strikes at random - in other words, there's nothing you can do to protect yourself other than vigilant screening.
It's true there are some risk factors you can't control. But scientists have long known the odds of developing breast cancer can be greatly reduced with the same kind of healthy living that also fends off many cases of heart disease, diabetes, and other cancers:
Eat a healthy diet. Get more exercise. Cut down on alcohol. Maintain a healthy weight, especially after menopause.
"Right now, there are things associated with lifestyle factors that one can absolutely control," said Susan Domchek, oncology professor and director of the MacDonald Women's Cancer Risk Evaluation Center at Penn Medicine.
This protection extends across many kinds of cancer. According to the American Cancer Society, adults who follow all healthy-lifestyle recommendations (including not smoking) are an estimated 36 percent less likely to be diagnosed with cancer and 40 percent less likely to die from it.
That's not to say that if you live virtuously you won't get cancer. These statistics refer to the entire population's chances, not just your individual risk. The point is that people with healthy habits are more likely to stay healthy.
"It's important to say what is attributable to an individual, and what is attributable across the population," said Domchek.
And some risk factors have more impact than others.
"We know that obesity, for example, is a significant risk for breast cancer across the population," she said.
Obesity is particularly dangerous after menopause, when the ovaries have stopped producing estrogen and estrogens arise primarily from fat tissue. Higher levels of estrogens mean a higher risk of breast cancer.
Although Domchek recommends a diet with plenty of fresh vegetables and fruits, she stops short of suggesting a specific eating plan.
Last month, a nearly five-year controlled study linked the Mediterranean diet - heavy in vegetables, fruits, whole grains, and olive oil and light in dairy products and red meat - to a reduction in breast cancer.
The study of 4,152 Spanish women ages 60 to 80 showed that those who consumed a Mediterranean diet supplemented with extra-virgin olive oil had a 68 percent lower risk of breast cancer when compared with groups whose diets were supplemented with nuts or who were advised to reduce dietary fat. The research was published in JAMA Internal Medicine.
Agreeing that this was good news, Domchek noted that it "can be difficult to separate out factors that prevent breast cancer, because we're looking at diet over a lifetime."
"The best diet," she said, "is a heart-healthy diet that gets you to a healthy weight."
Alcohol can also be a risk factor for breast cancer. According to Susan G. Komen for the Cure, which supports cancer research, women who down two to three drinks per day have a 20 percent increased risk of breast cancer. That's because alcohol can change how a woman's body metabolizes estrogen, increasing the amount of estrogen in the body.
"Cutting down on alcohol has been shown to be important," said Domchek. "If you drink one or two glasses of wine with dinner, you are increasing your risks."
Regular exercise also seems to reduce breast cancer 10 percent to 20 percent. The American Cancer Society recommends about 150 minutes of exertion per week; studies show that walking for about 30 minutes a day can reduce by 3 percent the chances of developing breast cancer. Aside from helping with weight control, there is some evidence that regular workouts might reduce estrogen in the bloodstream and boost the immune system, helping to ward off cancer cells.
"In addition," said Domchek, "exercise helps your heart, strengthens your bones, and improves your mood."
Though risk factors like obesity, alcohol use, and not exercising can be changed, Domchek stresses that a lot of breast-cancer contributors are harder to alter.
"In the U.S., children have periods earlier, we give birth to kids later, and we breast-feed less" than women in other countries, Domchek said. These reproductive risk factors all contribute to the higher levels of breast cancer in developed countries.
And the greatest risk factor is one no one can prevent: age. Most breast cancers occur in women older than 50.
When it comes to prevention, there are also medications that help with breast-cancer prevention for women in certain high-risk categories, said Domchek. They include Tamoxifen, which blocks actions of estrogen in breast tissue; and Raloxifine (Evista), which can treat osteoporosis and prevent breast cancer in postmenopausal women.
"Women may want to talk to their physicians or gynecologists to weigh the pros and cons of taking such medications," she said.
"This advice can feel a little bit dull," Domchek said. "But it's been shown over and over again that these things are really valuable. And diet and exercise affect many aspects of health - including heart disease, diabetes, and other diseases. They really are important."