A new study of survival rates shows that breast X-rays to detect cancer are about as effective in women under 50 as in older women, the American Cancer Society said. The study included 4,240 women with cancers found by mammography, a low-radiation X-ray that allows doctors to detect small tumors in the breast long before they can be felt in a physical examination. "We believe many lives can be saved . . . if mammography is used as a screening tool under age 50," said Dr. Arthur Holleb, Cancer Society senior vice president for medical affairs. Last week, an American Medical Assn. panel split on the question of mammography's effectiveness for symptom-free women under 50.