Clinical Context

Some patients argue that they smoke so little that they cannot possibly be at risk for cardiovascular or cancer outcomes. But what are the real attendant risks of smoking a minimal amount of cigarettes per day? Bjartveit and Tverdal previously assessed this issue in a study published in the October 2005 issue of Tobacco Control.^[1]

The study enrolled middle-aged men and women in Norway during the 1970s and followed them through 2002. The main study outcomes included cardiovascular mortality, cancer mortality, and overall mortality. Adults who reported smoking 1 to 4 cigarettes per day were compared with nonsmokers for these outcomes.

The risk for death resulting from cardiovascular causes was nearly 3-fold higher among light smokers vs never smokers. The respective relative risks for all-cause mortality among men and women were 1.57 (95% confidence interval [CI], 1.33-1.85) and 1.47 (95% CI, 1.19-1.82).

Light smoking did not confer a higher risk for mortality resulting from any cancer. However, light smoking was associated with a higher risk for lung cancer mortality among women, but not men.

It is understood that the risk for lung cancer declines after quitting smoking, but this process is poorly understood. The current study uses data from the Framingham Heart Study (FHS) to address this issue.

Study Synopsis and Perspective

Smokers who quit have a substantially lower risk for lung cancer than current smokers, even within 5 years of stopping smoking, new research shows.

"If you smoke, now is a great time to quit," says lead author Hilary Tindle, MD, MPH, the William Anderson Spickard Jr Professor of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee.

"The fact that lung cancer risk drops relatively quickly after quitting smoking, compared to continuing smoking, gives new motivation," she said in a statement.

However, former heavy smokers still have more than a 3-fold greater risk for lung cancer than those who never smoked for several decades after giving up the habit, the same research shows.

"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," said senior author Matthew Freiberg, MD, professor of medicine, Vanderbilt Center for Clinical Cardiovascular Outcomes Research and Trials Evaluation, Nashville, Tennessee.

The study was published online May 16 in the Journal of the National Cancer Institute.^[2]

Data from the FHS Original as well as the FHS Offspring cohort were analyzed for lifetime smoking and lung cancer incidence from 1954 to 1958 for the Original cohort and between 1971 and 1975 for the Offspring cohort. Lung cancer rates were tracked through 2013.

Information on smoking habits was collected and participants were categorized as current, former, or never smokers. Some 3905 participants from the FHS Original cohort and 5002 participants from the FHS Offspring cohort were included in the analysis.

"Most people (89.5%) who were smoking at baseline quit during follow-up and never relapsed," the authors point out.

However, during a median follow-up of 25.1 years for the FHS Original cohort and 33.6 years for the FHS Offspring cohort, investigators documented 284 diagnoses of lung cancer.

"Among ever smokers, the majority of lung cancers (92.7%) occurred among heavy smokers, with 21.3 or more cumulative pack-years of smoking," the researchers note.

In this subgroup of participants with 21.3 or more cumulative pack-years of smoking, the unadjusted lung cancer risk was greater than 10-fold higher compared with those who had never smoked.

Table. Incidence Rates of Lung Cancer per 1000 Person-Years

Persistently Elevated

Investigators also observed that the risk for lung cancer among long-term former smokers remained "persistently elevated," even 25 years after quitting.

For example, among those who had stopped smoking for less than 5 years, the risk for lung cancer was more than 12 times higher than it was for never smokers.

For the same long-term former smokers who quit smoking 5 to 9 years ago, the risk for lung cancer was still almost 12 times as high as it was for never smokers, whereas for those who hadn't smoked for 10 to 14 years, the risk for lung cancer was still almost 8 times higher than for never smokers.

For former heavy smokers who had stopped smoking 15 to 24 years ago, lung cancer risk was approximately 6 times higher than it was for those who never smoked, whereas for those who quit 25 years ago or more, the risk was more than 3-fold higher, at a hazard ratio (HR) of 3.85, the researchers point out.

Importantly, 4 of every 10 lung cancers diagnosed in former smokers in the current study occurred in individuals who had stopped smoking more than 15 years ago.

As such, these individuals do not qualify for lung cancer screening, the study authors emphasize.

Lung Cancer Screening

Commenting on the findings in a related editorial, Erin Hahn, MD, and Michael Gould, MD, both from Kaiser Permanente Southern California, Pasadena, discuss the implications of the study's findings as they relate to current lung cancer screening guidelines.^[3]

As they point out, multiple guidelines, including those from the US Preventive Services Task Force, recommend annual screening with low-dose computed tomography for people age 55 to 80 years who have a smoking history of 30 pack-years or longer and who currently smoke or who have quit smoking within the last 15 years.

Given these criteria, "a considerable percentage of current and former smokers who will receive a diagnosis of lung cancer may not be eligible for lung cancer screening," Dr Hahn and Dr Gould argue.

They also agree with the study authors that the persistently elevated risk for lung cancer in former heavy smokers as far out as 25 years after quitting is important.

"As the authors note, there are valid concerns about expanding current lung cancer screening eligibility," they acknowledge.

"Moreover, there is ongoing disagreement about whether the goal of a high-quality screening program is to identify the greatest number of treatable cancers or to maximize efficiency by providing screening services to those with the highest individual lung cancer risk," Dr Hahn and Dr Gould add.

The editorialists suggest that an "individualized" approach to lung cancer screening that takes into account multiple risk factors for lung cancer and not just smoking history might be the better way for physicians to make sure patients are appropriately screened, even though such an approach may not be covered by insurance policies.

The study authors have disclosed no relevant financial relationships. Dr Gould reports receiving research support from Medial EarlySign to develop computer models of lung cancer risk. Dr Hahn has disclosed no relevant financial relationships.

J Natl Cancer Inst. Published online May 16, 2018.

Clinical Implications

• A previous study demonstrated that light smoking was associated with higher mortality rates as a result of cardiovascular disease or any cause, but light smoking did not significantly increase the risk for overall cancer mortality.
• The current study finds that the risk for lung cancer declines fairly quickly after cessation from smoking, but the risk for lung cancer remains elevated in former heavy smokers compared with never smokers, even after more than 25 years of tobacco cessation.
• Implications for the Healthcare Team: The current study emphasizes the power of smoking cessation in cancer prevention, but it may be even more persuasive in delineating the benefit of cancer prevention with avoiding any smoking in the first place.

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