Diagnoses of breast cancer, the disfiguring disease most dreaded by American women, rose steadily during the 1980s and now appear to have leveled off. But death rates from the disease have changed little since the 1930s. To some experts, the data indicate that modern medicine is conquering the disease through treatments like chemotherapy and regular screening with mammograms,which can catch the disease at an earlier, more curable stage. But to skeptics, the statistics mean that breast cancer remains an intractable disease and that scientists have yet to find a cure for its most virulent forms. As politically savvy breast cancer activists push for increased federal spending to find a cure, they are asking increasingly skeptical questions about how the money should be spent.