Quitting smoking early was associated with higher survival rates following a lung cancer diagnosis, according to a study.

The study, led by researchers at Harvard T.H. Chan School of Public Health, showed that compared to those who never smoked and were being treated for non-small cell lung cancer (NSCLC), current smokers had 68 per cent higher mortality and former smokers had 26 per cent higher mortality.

"Our participants' smoking histories varied, with some having stopped smoking a few years before their diagnosis and others having stopped several decades before," said senior author David Christiani, Professor of Environmental Genetics.

"This wide range gave us confidence in our results -- that the benefit of pre-diagnosis smoking cessation persists even after lung cancer is diagnosed."

The study, published online in JAMA Network Open, followed 5,594 patients with NSCLC -- which accounts for 85 per cent of all lung cancer cases -- enrolled between 1992 and 2022. Of these participants, 795 had never smoked; 3,308 were former smokers; and 1,491 were current smokers.

During the study period, 3,842 of the participants died: 79.3 per cent of the current smokers, 66.8 per cent of former smokers, and 59.6 per cent of the never smokers.

While never smoking was associated with the best odds of survival after a lung cancer diagnosis, the findings showed significant associations between lower mortality and having quit smoking pre-diagnosis.

The longer a patient went without smoking, the more health benefits they accrued: For former smokers, doubling the years of smoking cessation before their lung cancer diagnosis was significantly associated with prolonged survival.

Conversely, doubling smoking-pack years was associated with shorter survival among current and former smokers diagnosed with NSCLC.

The researchers noted that associations between survival and smoking history may vary depending on the clinical stage at which lung cancer was diagnosed and that the study did not account for the different kinds of treatment participants were receiving.