However, quitters who were older or who had earlier stages of lung cancer did not have an advantage, said the researchers, whose findings appeared in the journal Cancer.

It’s known that people who never smoked are more likely to survive the disease, but whether former smokers do better than current ones has been less clear.

“The findings do suggest there is some benefit to quitting smoking,” said Amy Ferketich of Ohio State University College of Public Health in Columbus, who worked on the study.

Her group used medical records from 4200 lung cancer patients treated at eight cancer centers around the country.

Patients who never smoked were more likely to survive the less advanced cancers — stage 1, 2 or 3 — than were former or current smokers, the researchers found.

Among smokers with stage 1 or 2 lung cancer, for instance, 72% survived at least two years, compared to 93% of the never-smokers and 76% of people who’d kicked the habit a year or more before diagnosis.

Only 15% of smokers with stage 4 disease survived two years, while 40% of never-smokers and 20% of former smokers did.

After adjusting the numbers for factors such as age, race and radiation treatment, the researchers determined that quitters were just as likely to die from the early-stage cancers as were current smokers.

But for advanced cancers, people under 85 who had stopped smoking more than a year before their diagnosis survived longer than smokers. Forty-five-year-old former smokers, for instance, were 30% less likely to die from stage 4 lung cancer within two years than were current smokers.

Smoking is the number one risk factor for developing lung cancer, and studies have shown that people who quit are less likely to get it than current smokers, but it’s not clear why smokers already diagnosed with lung cancer fare worse than non-smokers, Ferketich said.

“In general, never smokers are healthier individuals, so they tend to, in a lot of trials, have better outcomes with disease than people who continue to smoke,” she said.

“Just the continued exposure to tobacco might make the disease progress more quickly in smokers compared to never-smokers who don’t have that exposure.”

Ferketich said it’s also possible that smoking could influence the biology of the cancer, and perhaps smokers get tumors that never-smokers are less likely to develop. She added that it’s never too late to quit.