WASHINGTON How long and how much you smoked, and how long it's been since the last puff, make a difference in the risk of getting lung cancer.
Scientists have come up with a formula that certain smokers and ex-smokers can use to calculate that risk one that could help people decide if they really want a controversial test for lung cancer.
The formula, published in this week's Journal of the National Cancer Institute, shows a wide variation in risk.
Consider a 51-year-old woman who smoked a pack a day since she was 22 until stopping nine years ago. The formula puts her chances of getting lung cancer in the next 10 years at less than 1 in 100.
Compare a 68-year-old man who smoked two packs a day since he was 18 and hasn't yet quit. He has a 1 in 7 chance of lung cancer by his 78th birthday if he keeps puffing. If he quit smoking today, the risk drops slightly, to 1 in 9.
The formula only works for certain people those older than 50, who smoked at least half a pack a day for at least 25 years because it's based on a study that tracked cancer development in just those people.
Researchers from New York's Memorial Sloan-Kettering Cancer Center created the formula and posted a version on the center's Web site Tuesday.
Doctors have used a similar model for years that calculates age, family medical history and other factors to predict a woman's risk of getting breast cancer.
But for lung cancer, expected to kill 157,000 Americans this year, doctors could give only vague advice: Smoking is the chief cause; heavy smokers have the highest risk; and that risk drops with each year that passes since kicking the habit.
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