• U.S.

David Bjerklie

Smoking seems to bring out the inner statistician in people. Sure, smokers know their habit can lead to lung cancer, but what are the odds it actually will? How does smoking a pack a day for 20 years compare in risk with smoking two packs daily for 40 years? And if you quit, how much do your odds improve? The results of a study published last week in the Journal of the National Cancer Institute offer smokers some help–at least with the math. The tricky part is knowing what to do with the answer.

As you might expect, the risk of lung cancer varies according to when you start to smoke, how long and how much you smoke, and when you quit. By tracking the incidence of the disease in 18,172 men and women ages 50 to 69 who had been or still were heavy smokers, researchers at New York City’s Memorial Sloan-Kettering Cancer Center and Seattle’s Fred Hutchinson Cancer Research Center devised a mathematical model that predicts the likelihood that lung cancer will be diagnosed in a smoker within 10 years. You can find the model on the Web at www.mskcc.org

Consider the case of a 51-year-old woman who smoked a pack a day from age 14 until she stopped at age 42. The model puts her chances of getting lung cancer in the next decade at less than 1 in 100. Compare that with a 68-year-old man who has smoked two packs a day for 50 years and hasn’t quit. He has a 1-in-7 chance of getting lung cancer by his 78th birthday. If he quits, his 10-year risk drops to 1 in 9.

So what’s a smoker to think? A 1-in-7 chance of getting lung cancer will scare some folks into quitting, but you might be tempted to shrug off a 1-in-100 chance and think to yourself, As long as I quit by 42, I’m O.K. Think again. More smokers die of heart disease than lung cancer–not to mention that smokers have greater susceptibility to emphysema and other chronic illnesses.

A better idea would be to use the risk model to help decide whether to undergo an experimental screening procedure called low-dose spiral computed tomography. More and more hospitals are offering this scan in the hope of catching lung cancer early, thereby improving a fairly dismal survival rate. The scan can detect nodules too small to be seen on conventional X rays.

So far so good. But the scan also spots a lot of things that are not lung cancer. “In up to half of patients screened, the scan will show some lung scar or shadow that will require further evaluation or surgical biopsy, even though it will ultimately be deemed to be harmless,” says Dr. Peter Bach of Memorial Sloan-Kettering. The best candidates for the scan, therefore, are those who already have the highest risk–and the most to gain.

Of course, no model, no matter how sophisticated, is foolproof. And this one won’t tell you who should get a lung scan. That’s something smokers and ex-smokers should decide with their doctors. And if you need any more reasons to quit, consider this: in some ways, a little smoking may be as bad as a lot. Researchers looking at the lining of blood vessels were surprised to find the same damage whether the subjects smoked a pack a day or a pack a week. When it comes to smoking, the odds are always against you.