People who have smoked an average of 1 pack a day for 20-39 years tripled their tumor risk versus less-heavy smokers and 30-fold versus those who never smoked. For those who smoked the equivalent of 1 pack for 40-60 years, or 2 packs for 20-30 years, the risk levels grew by fourfold and 40-fold, respectively. For those who’ve smoked even more, the likelihood of developing lung cancer is high, but the risk remains stable and doesn’t grow more over time, according to the analysis.
The report, released at the annual European Lung Cancer Congress 2023 meeting, and an earlier related study “underscore the importance of smoking abstinence and early smoking cessation,” said study lead author J. Anthony Nations, MD, MBA, in an interview.
The earlier study, published in JAMA Oncology, relied on a “pack-year” analysis to evaluate the risk of lung cancer in smokers. A pack-year refers to the cigarette use of a person who smoked a pack a day for 1 year. It’s the equivalent of smoking half a pack for 2 years or 2 packs for 6 months.
By this measure, a smoker with 20 pack-years of cigarette use smoked the equivalent of a pack a day for 20 years or 2 packs a day for 10 years. U.S. guidelines recommend annual low-dose CT lung cancer screening in adults who are aged 50-80, have more than 20 pack-years of tobacco exposure, and either currently smoke or quit within the last 15 years.
The JAMA Oncology report “showed that, compared with never-smokers, current heavy and nonheavy smokers had [a] 40 and 10 times higher risk of lung cancer, respectively,” said Dr. Nations, who is also a pulmonologist with Washington D.C. Veterans Affairs Medical Center. “A smoking history of greater than 20 pack-years was considered heavy, but current heavy smokers had a median pack-year smoking history of 50 pack-years. This observation prompted us to want to look more closely at pack-year smoking history.”
For the new analysis, researchers tracked 2,505 older adults (mean age, 73 ± 5.7 years; 69% women, 17% African American) in the Cardiovascular Health Study. Of those, 532 were current smokers (18% less than 20 pack-years, 30% 20-39 pack-years, 34% 40–59 pack-years, and 18% greater than 60 pack-years).
Lung cancer occurred in 0.5% of those who never smoked, 5% of those who smoked less than 20 pack-years, 14.6% of those who smoked 20-39 pack-years, 17.7% of those who smoked 40-59 pack-years, and 16.0% for those who smoked more than 60 pack-years. In an analysis adjusted for age, sex, race, and competing risk of death, researchers found that those who smoked less than 20 pack-years were 9.73 times more likely to develop lung cancer than those who never smoked (hazard ratio, 9.73). The HRs of lung cancer versus never-smokers for the other groups were 30.33 (20-39 pack-years), 42.97 (40-59 pack-years), and 46.02 (greater than 60 pack-years.).
“While it was not surprising that the risk of lung cancer in current heavy smokers would be proportionately greater in smokers with higher pack-year smoking history, we were surprised to see that the risk almost plateaued in the heaviest current smokers,” Dr. Nations said.
As for the clinical message from the findings, Dr. Nations said they reveal that quitting smoking makes a difference in lung cancer risk, even after many years of heavy smoking. “Smokers who quit after a 30–pack-year smoking history will not incur the higher risk of those with a 40– or 50–pack-year smoking history.”
The previous JAMA Oncology paper also showed that quitting pays dividends by reducing lung cancer risk. Subjects with at least 20 pack-years of smoking who quit less than 15 years ago nearly halved their excess risk of lung cancer, compared with similar current smokers who didn’t quit.
In an interview, cancer researcher Robert J. Volk, PhD, of the University of Texas MD Anderson Cancer Center, Houston, praised the new analysis but noted that it has limitations: “The sample is fairly small – 532 adults who currently smoke – and the subgroups based on pack-years are even smaller.”
No study funding is reported. The study authors and Dr. Volk reported no disclosures.
*This article was updated on 4/17/23.