Smokers and ex-smokers can predict their risk of lung cancer by using a mathematical formula.
Smoking causes lung cancer
The formula takes into account how long and how much they smoked, and, in the case of ex-smokers, how long it has been since they quit.
The calculation shows a wide variation in risk:
The formula works only 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.
- A 51-year-old woman who smoked a pack a day since she was 14 until stopping nine years ago is rated as having less than a one in 100 chance of developing lung cancer.
- However, a 68-year-old man who smoked two packs a day since he was 18 and hasn't yet quit has a one in seven chance of lung cancer by his 78th birthday if he continues smoking. If he quit immediately, his risk would fall to one in nine.
It has been created by researchers from New York's Memorial Sloan-Kettering Cancer Center.
They have posted an easy-to-use version for consumers on the web.
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.
However, until now they have been unable to be so precise about predicting lung cancer risk because it varies depending on various different factors.
The new formula would help doctors to be more specific.
Researcher Dr Peter Bach said: "The risk assessment tool should help physicians and patients balance the possible risks and benefits of screening.
"For example, the subjects in our study who were at the low end of risk had less than a 1% chance of getting lung cancer in the next 10 years.
"That needs to be contrasted with the 30% to 50% risk that a screening CT will show some lung scar or shadow that requires further evaluation or surgical biopsy, even though it will ultimately be deemed to be harmless."
The formula is published in the Journal of the National Cancer Institute.