WASHINGTON — How long and how much you smoked, and how long it's been since the last puff, make a difference in your risk of 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 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.
The new formula will help doctors "be more specific now about who is at greatest risk," said Dr. Tom Glynn of the American Cancer Society, who praised the research.
That's particularly important as more people consider getting those aggressively advertised, but still unproven, spiral CT scans to hunt early lung cancer, Glynn said.
Only 15 percent of lung cancer sufferers survive five years, mainly because the disease usually is diagnosed very late. There is no proven screening test so far.
The National Cancer Institute is studying whether spiral CT scans, which view the lungs at various angles, could improve survival by spotting tumors early. There's no answer yet, and the scans do have a big problem: Up to half detect harmless scar tissue or some other benign lump that requires a risky biopsy or other follow-up testing.
Lung specialists see many patients "wracked by anxiety and concern about what may be in their future" because of ambiguous CT results, said Dr. Peter Bach, Sloan-Kettering's lead researcher. "A lot of chest physicians, I believe, would welcome a way of helping patients up front decide whether they should have this test in the first place."
First, Bach had to prove there is measurable variation in risk. He turned to the Fred Hutchinson Cancer Research Center in Seattle, which in the 1990s performed one of the best studies ever to track lung cancer development in 18,000 heavy smokers and ex-smokers. Bach used that data to determine the effects of age, sex, smoking history and exposure to cancer-causing asbestos.
He created a model that, while not perfect, largely accurately predicted cancer development among the Hutchinson study participants and among people being screened for lung cancer at the Mayo Clinic.
It's not foolproof, Bach cautioned. Nor does the formula say whether a person should have a CT scan.
Instead, people will have a prediction of risk based on data that they can use to make health-care decisions, agreed the cancer society's Glynn — who encouraged users of the Web site to discuss the prediction with their doctor to ensure they interpret it correctly.
Some people will find a 15 percent risk of cancer so worrisome that they race for a CT scan, Glynn noted, while others might say, "'That's 1 in 7, and I'm going to be one of the six" who stays well.
On the Net: Risk formula: www.mskcc.org/PredictionTools/LungCancer