Risk of Incident Lung Cancer Drops by 39% Five Years After Smokers Quit

A new analysis of findings from the Framingham Heart Study by researchers at the Vanderbilt University Medical Center suggests that a smoker’s risk of lung cancer drops substantially within 5 years of quitting.

The analysis, published in the Journal of the National Cancer Institute, evaluated 3905 participants in the original Framingham cohort who attended their fourth examination cycle (1954 to 1958) and 5002 participants in the offspring cohort who attended their first examination cycle (1971 to 1975). Participants were followed until the end of 2013 for development of lung cancer, and smoking status was ascertained every 2 years and every 4 years in the 2 cohorts, respectively.

The researchers found that most people (89.5%) who were smoking at baseline quit during the follow-up period and did not begin smoking again. The median abstinence period was 2.9 years (range, 0.6 years-39.7 years).

During follow-up, there were 284 cases of incident lung cancer. The majority of lung cancers (92.7%) occurred among heavy smokers, with 21.3 or more cumulative pack-years of smoking (a pack-year is a measure by which the number of packs of cigarettes smoked per day is multiplied by the number of years a person has smoked).

After adjusting for age, sex, education level, decade of examination, and alcohol consumption, the risk of lung cancer was 39.1% lower among those who were previously heavy smokers and who had quit than it was for current smokers, and this lower risk was detectable within 5 years of quitting. Furthermore, risk continued to fall with an increasing number of years since quitting.

A secondary analysis, with further adjustment for cumulative pack-years, confirmed the lower risk of lung cancer among heavy former smokers than among current smokers, though long-term former smokers still had an elevated lung cancer risk at 25 years compared with those who had never smoked.

“If you smoke, now is a great time to quit,” said lead author Hilary Tindle, MD, MPH, of the Vanderbilt University School of Medicine and director of the Vanderbilt Center for Tobacco, Addiction, and Lifestyle, in a statement. “The fact that lung cancer risk drops relatively quickly after quitting smoking, compared to continuing smoking, gives new motivation,” she added.

Yet the analysis did not yield only positive news for smokers who are concerned about lung cancer; the researchers found that only 58.7% (152) of cases of lung cancer among both former and current smoker met screening edibility criteria at the time of diagnosis, and 40.8% of diagnoses of lung cancer occurred among former smokers with more than 15 years since quitting (a duration that is beyond the window of current screening eligibility). The authors call for further investigation of whether the 15-year threshold should be removed in order to maximize benefit.

“While the importance of smoking cessation cannot be overstated, former heavy smokers need to realize that the risk of lung cancer remains elevated for decades after they smoke their last cigarette, underscoring the importance of lung cancer screening,” added senior author Matthew Freiberg, MD, MSc, also of Vanderbilt University Medical Center.

Reference

Tindle HA, Duncan MS, Greevy RA, et al. Lifetime smoking history and risk of lung cancer: results from the Framingham Heart Study. JNCI J Natl Cancer Inst. 2018;110(11):djy041. doi: 10.1093/jnci/djy041.