"What can be achieved with screening has been achieved. We can't do much more," Carlo La Vecchia, head of epidemiology at the University of Milan, said in an interview. "It's time to move on to other things."
La Vecchia spoke Thursday at a European breast cancer conference in Barcelona. He cited figures from the International Agency for Research on Cancer, which estimates that 25 to 30 percent of breast cancer cases could be avoided if women were thinner and exercised more. The agency is part of the World Health Organization.
His comments are in line with recent health advice that lifestyle changes in areas such as smoking, diet, exercise and sun exposure can play a significant role in risk for several cancers.
Dr. Michelle Holmes of Harvard University, who has studied cancer and lifestyle factors, said people might wrongly think their chances of getting cancer depend more on their genes than their lifestyle.
"The genes have been there for thousands of years, but if cancer rates are changing in a lifetime, that doesn't have much to do with genes," she told The Associated Press in a phone interview from Cambridge, Massachusetts.
Breast cancer is the most common cancer in women. In Europe, there were about 421,000 new cases and nearly 90,000 deaths in 2008, the latest available figures. The United States last year saw more than 190,000 new cases and 40,000 deaths.
A woman's lifetime chance of getting breast cancer is about one in eight. Obese women are up to 60 percent more likely to develop any cancer than normal-weight women, according to a 2006 study by British researchers.
Many breast cancers are fueled by estrogen, a hormone produced in fat tissue. So experts suspect that the fatter a woman is, the more estrogen she's likely to produce, which could in turn fuel breast cancer. Even in slim women, experts believe exercise can help reduce the cancer risk by converting more fat into muscle.
Yet any discussion of weight and breast cancer is considered sensitive because some may misconstrue that as the medical establishment blaming women for their disease.
Tara Beaumont, a clinical nurse specialist at Breast Cancer Care, a British charity, said her agency has always been careful about giving lifestyle advice. She noted that three of the major risk factors for breast cancer -- gender, age and family history -- are clearly beyond anyone's control.
"It is incredibly difficult to isolate specific factors. Therefore women should in no way feel that they are responsible for developing breast cancer," she said.
Yet Karen Benn, a spokeswoman for Europa Donna, a patient-focused breast cancer group, said it was impossible to ignore the increasingly stronger links between lifestyle and breast cancer.
"If we know there are healthier choices, we can't not recommend them just because people might misinterpret the advice and feel guilty," she said. "If we are going to prevent breast cancer, then this message needs to get out, particularly to younger women."
That means avoiding becoming overweight as an adult. Robert Baan, a cancer expert with the international cancer research agency, said it isn't clear if women who lose weight can lower their risk to the level of a woman who was never fat.
The American Cancer Society Web site says the connection between weight and cancer risk is complex. It says risk appears to increase for women who gain weight as adults, but not for women who have been overweight since childhood. The cancer society recommends 45 to 60 minutes of physical activity five or more days a week to reduce the risk of breast cancer.
Drinking less alcohol might also help. Experts estimate that having more than a couple of drinks a day can boost the risk of breast cancer by 4 to 10 percent.
After studies several years ago linked hormone-replacement therapy to cancer, millions of women abandoned the treatment, leading to a sharp drop in breast cancer rates. Experts said a similar reduction might be seen if women ate healthier and exercised more.
Holmes, the Harvard expert, said changing diet and nutrition is arguably easier than tackling other breast cancer risk factors.
In the 1980s and 1990s, breast cancer rates steadily increased, paralleling a rise in obesity and the use of estrogen-containing hormones after menopause.
La Vecchia said countries like Italy and France -- where obesity rates have been stable for the past two decades -- show that weight can be controlled at a population level.
"It's hard to lose weight, but it's not impossible," he said. "The potential benefit of preventing cancer is worth it."