When do you stop lung cancer screening after quitting smoking?

Most of the confusion comes from treating lung cancer screening as one rule. There are four that matter in the US, written by bodies with different jobs: the USPSTF sets the federal preventive-care standard, ACS and CHEST are professional guideline groups, and CMS decides what Medicare pays for. They differ on age and pack-years at the edges, and most sharply on the question in the title.

Pack-years are packs per day multiplied by years smoked. ACS gives the examples: one pack a day for 20 years is 20 pack-years, and so is two packs a day for 10 years.

The Task Force's 2021 recommendation puts the stopping rule in the recommendation itself: "Screening should be discontinued once a person has not smoked for 15 years or develops a health problem that substantially limits life expectancy or the ability or willingness to have curative lung surgery." The 2021 update widened who starts (50 instead of 55, 20 pack-years instead of 30). The 15-year stop rule is carried over unchanged from 2013.

The number matches the trial evidence. The two largest trials behind screening set their own quit limits: NLST, with 53,454 participants, enrolled people who "were current smokers or had quit within the past 15 years," and NELSON enrolled people who had quit within the past 10 years. Fifteen years smoke-free is the edge of what the largest trial tested. The Task Force's reasoning about risk runs the same way: lung cancer risk "increases with cumulative quantity and duration of smoking and with age but decreases with increasing time since quitting for persons who formerly smoked."

The American Cancer Society reached the opposite conclusion on the same question. Its 2023 guideline, published in CA: A Cancer Journal for Clinicians, states: "For individuals who formerly smoked, the number of YSQ [years since quitting] is not an eligibility criterion to begin or to stop screening." To get there its guideline group drew on a systematic review of lung cancer risk by years since quitting and an epidemiologic and modeling analysis of how quitting and aging affect that risk.

ACS's announcement, dated November 1, 2023, calls this "the most important change in the updated guideline" and explains it in one line: "many people who started screening will not lose eligibility by reaching 15 years since they quit. This is important because their risk of lung cancer is still rising due to increasing age." In other words, quitting lowers risk, aging raises it, and ACS judged that for someone with a 20 pack-year history the second effect does not let the risk fall low enough at 15 years to stop.

Two consequences follow, both stated by ACS. A former smoker "whether they quit yesterday or 20 years ago" counts as high risk if the age and pack-year criteria are met. And people with 20 pack-years who "reach their 15th year since quitting before age 50 will now be eligible to start and continue screening" once they reach 50, a group the USPSTF rule never admits.

ACS also widened the age band, from 55 to 74 under its previous guideline to 50 to 80 now, and its guideline abstract adds that "inferential evidence supports LCS for men and women older than 80 years who are in good health." Its recommendation itself still stops at 80.

The American College of Chest Physicians published its guideline in 2021, the same year as the USPSTF, and keeps the 15-year limit in both of its eligibility bands. Its fourth recommendation is explicit: for people who "have quit smoking more than 15 years ago, and are not projected to have a high net benefit from lung cancer screening based on clinical risk prediction or life-year gained calculators, we recommend that low dose CT screening should not be performed."

The qualifier matters. Its third recommendation suggests offering screening to people outside the standard criteria who "are projected to have a high net benefit" on validated risk prediction or life-year-gained calculators. So under CHEST, passing 15 years smoke-free closes the default route and leaves a calculated one open.

Medicare's national coverage determination, effective February 10, 2022, follows the USPSTF on quitting and stops earlier on age. A beneficiary must be 50 to 77, have no signs or symptoms of lung cancer, have at least 20 pack-years, be a "current smoker or one who has quit smoking within the last 15 years," and have an order for the scan. Before the first scan, Medicare requires a counseling and shared decision-making visit. The Part B coinsurance and deductible are waived.

Coverage is a separate question from a recommendation, and CHEST says so directly: "In the United States, health insurance providers may not pay for low-dose CT screening for those who do not meet the eligibility criteria" in its first two recommendations. A former smoker who is eligible under ACS at 17 years smoke-free can still find the scan is not covered.

Three hypothetical readers, each run through the four rules exactly as published. This is our application of each body's criteria, with no other inputs.

The middle row is the whole disagreement in one person. The USPSTF says stop, ACS says continue, CHEST says stop unless a calculation says otherwise, and Medicare does not pay.

Three things are not in dispute. Screening is for people without symptoms. ACS puts it plainly: "If you have symptoms that could be from lung cancer, see your doctor right away," and testing done for symptoms "is for diagnosis and is not the same as screening," so none of these eligibility rules apply to it. Screening ends when health changes: the USPSTF names a health problem that limits life expectancy or the ability or willingness to have curative lung surgery, ACS says people with conditions that substantially limit life expectancy "should not be screened," and CHEST recommends against screening when comorbidities both substantially limit life expectancy and limit the ability to tolerate evaluation or treatment. And screening does not replace quitting; in ACS's words, "Screening is not a good alternative to stopping smoking."

Our lung cancer screening page carries the full eligibility rows for USPSTF, ACS and CHEST, each quoted and dated, along with how the scan is done and what happens after an abnormal result. Your personal screening plan lists the screenings that apply at your age and sex.

If you are approaching 15 years smoke-free, it is worth raising before your next scan rather than after it. Useful questions for your clinician: which body's criteria does this practice follow, does my insurance follow the same rule, and would a risk calculator change the picture for me. The decision to continue or stop is made with a clinician who knows your full history.

Former and current smokers have other prevention dates that move with smoking history. Smoking is one of the conditions that brings the pneumococcal vaccine forward from its routine age, covered in the vaccines recommended after 50, and the blood tests worth getting every year covers the routine numbers that sit alongside screening.