WASHINGTON (AP) – Scientists may have found a way to tell which smokers are at highest risk of developing lung cancer: measuring a telltale genetic change inside their windpipes.

A test based on the research now is being developed, in hopes of detecting the deadly cancer earlier, when it is more treatable.

If the work pans out, the next big question is: Might it even be possible to reverse this genetic chain reaction before it ends in full-blown cancer? The researchers found a tantalising early hint among a handful of people given an experimental drug.

"They're heading towards lung cancer, and we can identify them with this genomic test," said Dr. Avrum Spira of Boston University School of Medicine, who led the research published on Wednesday in the journal Science Translational Medicine.

Lung cancer – which killed nearly 160,000 Americans last year – is the leading cancer killer, and cigarette smoke is by far its main cause. Yet, not all smokers develop lung cancer; Spira cites estimates that ten to 20 percent will. The risk depends in part on how much people smoke, for how long and how long ago they quit, but there is no way to predict who will escape it and who will not. Nor is there a good way to detect early-stage tumours. Consequently, most people are diagnosed too late for today's treatments to do much good.

"Even for people who have stopped smoking, there's a significant risk of cancer down the road, and it would be nice to identify which patients are really at risk," said Duke University lung cancer specialist Dr. Neal Ready, who was not involved with the new research but called it intriguing.

"To have some sort of molecular test that would identify those patients would be very useful," he added, because they could receive close monitoring with lung scans that are not practical for everybody.

Rather than focusing on the lung itself, Spira's team took a different approach. Smoking bathes the entire respiratory tract in toxins. So he hunted for the earliest signs of impending lung cancer upstream, in how different genes are turned on and off inside the upper airway as the body tries to defend itself and those defences weaken over time. Every year at least 100,000 smokers or former smokers have tubes snaked down their throats to look for signs of cancer if an X-ray or other test detects something suspicious, Spira explained. A bronchoscopy can be used for a look into the lung, but Spira was interested instead in cells that line the windpipe, collected during the same procedure.