ST. PETERSBURG, FLA. — ST. PETERSBURG, Fla. -- In a major shift of its goals, the American Cancer Society says that instead of waiting for cures, it will refocus major resources on preventing cancer through dietary modifications. Promising research showing that some foods contain chemicals that protect people against cancer while others promote cancer formation has opened the door to massive efforts to prevent cancer, said Dr. Walter Lawrence Jr., cancer society president and professor of surgery at the Medical College of Virginia. He spoke yesterday at the society's annual Science Writers Seminar. To back up its new commitment, the cancer society announced it had launched two pilot studies to determine if low-fat diets will reduce the risk of breast cancer, which kills 46,000 American women annually, and if high-fiber diets will reduce the risk of colon cancer, which will claim the lives of 58,000 Americans this year. If the pilot studies are successful, large-scale studies will be initiated across the country, studies that even the federal government balked at undertaking because of their prohibitive cost.