Hospitals and radiology practices around the United States have increasingly begun using high-tech CT scans to screen people for lung cancer despite intense disagreement about whether the testing should be done widely and, if so, who should undergo the exams. The trend was triggered by a recent federal study, which found that screening certain heavy smokers and ex-smokers could slash their chances of dying from lung cancer. The finding was hailed as one of the most important advances in decades toward reducing the toll from the nation's leading cancer killer. Proponents of screening say the scans could save thousands of lives, and people at risk of lung cancer should be able to get the exams -- and insurance companies should pay for them -- in consultation with their doctors. Critics of wide-scale testing acknowledge that CT screening represents an exciting advance but argue that it remains far from clear whether the benefits will outweigh the risks in the real world.