The short answer
Annual low-dose CT cuts lung cancer deaths by about 20 percent in eligible people. It also finds many small nodules, nearly all benign, and findings outside the lungs.
The USPSTF recommends annual low-dose CT for adults 50 to 80 who have a 20 pack-year smoking history and currently smoke or quit within the past 15 years. It is a Grade B recommendation.
In the National Lung Screening Trial, annual low-dose CT reduced lung cancer deaths by about 20 percent compared with chest X-ray.
Most scans are reassuring. Roughly 90 percent of baseline screening CTs fall into Lung-RADS categories 1 or 2 and simply return in 12 months.
Using Lung-RADS reporting, the baseline false-positive rate is about 12.8 percent, down from 26.6 percent under the original NLST criteria.
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The full explanation.
Who screening is for
The USPSTF recommends annual low-dose CT for adults aged 50 to 80 who have at least a 20 pack-year smoking history and either still smoke or quit within the past 15 years. It carries a Grade B, meaning the Task Force found moderate net benefit and recommends offering it.
A pack-year is one pack a day for one year. Half a pack a day for 40 years counts the same as two packs a day for 10. Work out your own total before the appointment, because eligibility and insurance coverage both turn on that number.
The recommendation also says when to stop: once you have not smoked for 15 years, or once a health problem would prevent you from tolerating curative lung surgery. Screening only helps if a found cancer can be treated.
What the evidence shows
The National Lung Screening Trial found roughly a 20 percent reduction in lung cancer deaths with annual low-dose CT compared with chest X-ray. The European NELSON trial produced a similar direction of effect, with a lung cancer incidence rate ratio of 0.75 at 10 years. Lung CT is one of the better-performing screening tests available, and estimated overdiagnosis is low at around 6 percent of screen-detected cases.
Most scans find something, and most of it is nothing
Small lung nodules are extremely common, especially in people who have smoked. Scars from old infections, healed inflammation, and small lymph nodes all show up as spots.
The Lung-RADS system exists to keep those spots from turning into panic. About 90 percent of baseline screening CTs land in category 1 or 2, both of which mean come back in 12 months. Roughly 5 percent are category 3, probably benign, with a 6-month repeat scan. About 2 percent each are 4A and 4B, which move to shorter intervals or diagnostic workup.
Using Lung-RADS to define what counts as positive, the baseline false-positive rate is about 12.8 percent, compared with 26.6 percent under the older NLST definition. That improvement is the single biggest change in how screening feels to go through.
Two rulebooks, and knowing which one applies
This trips up a lot of people who read about nodules online.
If a nodule is found on a lung cancer screening CT, it is managed under Lung-RADS. The population is high-risk by definition and the thresholds reflect that.
If a nodule is found incidentally, on a CT ordered for chest pain, trauma, an abdominal problem, or before surgery, it is managed under the Fleischner Society criteria instead. Those criteria explicitly do not apply to people under 35, people who are immunosuppressed, people with a known cancer, or people in a screening program. Under Fleischner, a single solid nodule under 6 mm in a low-risk person needs no routine follow-up at all, a threshold that surprises people who expect every spot to be chased.
If you are trying to make sense of a report, the first question is which system was used. A category number means Lung-RADS. A recommendation phrased purely in millimeters and months usually means Fleischner.
The costs of screening, stated plainly
Findings outside the lungs. The scan covers the thyroid, heart and coronary arteries, aorta, esophagus, liver, adrenal glands, and spine. Incidental findings appear in 4 to 41 percent of screened people, flagged with an S modifier. Coronary artery calcification is the most common and is often worth acting on. Others lead to workups that end in nothing.
Radiation. A single low-dose CT delivers about 0.65 to 2.36 mSv. Average annual background radiation is about 2.4 mSv. The dose is small per scan and accumulates across years of annual screening, which is part of why eligibility is limited to people whose lung cancer risk is high enough to justify it.
Procedures. A minority of participants go on to biopsy or surgery, and a minority of those turn out not to have cancer.
Making screening work
Screening is a program, not an event. The benefit in the trials came from annual scans over years, with follow-up completed on schedule. The most common failure is a 6-month or 3-month recommendation that quietly never gets booked.
Before you leave, write down the category, the exact next date, and who is responsible for reminding you. If you still smoke, ask about cessation support at the same visit; quitting lowers lung cancer risk more than any scan can.
Sources
Words to know
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Common questions
I do not meet the pack-year threshold but I am worried. Can I still get screened?
The benefit-to-harm balance behind the recommendation was measured in people who meet the eligibility criteria, and it does not automatically carry over to lower-risk groups, where the same false positives and incidental findings occur while fewer cancers are found. Insurance coverage generally follows the USPSTF and CMS criteria as well. If you have other exposures such as radon, asbestos, or a strong family history, raise those specifically; they can change the conversation even though they are not part of the formal criteria.
What is a pack-year and how do I calculate mine?
One pack-year is smoking one pack a day for one year. Multiply packs per day by years smoked. Half a pack a day for 40 years is 20 pack-years, and two packs a day for 10 years is also 20. Add up separate periods if your smoking changed over time. Bring the number to the appointment, since eligibility hinges on it.
If I quit smoking 20 years ago, why am I no longer eligible?
Risk declines steadily after quitting, and beyond about 15 years the chance of finding a lung cancer falls enough that the harms of annual CT, mainly false positives, workups, and incidental findings, start to outweigh the benefit for the group as a whole. The USPSTF also advises stopping screening once someone has not smoked for 15 years or has health problems that would prevent curative lung surgery.
The report mentioned something not in my lungs. What happens now?
Low-dose CT of the chest also images the thyroid, heart and coronary arteries, aorta, esophagus, liver, adrenal glands, spine, and part of the breast tissue. Findings outside the lungs appear in 4 to 41 percent of scans and are flagged with an S modifier on the Lung-RADS report. Most need nothing beyond a note. Coronary artery calcification is the most common and is genuinely useful information about heart risk. Ask specifically whether the finding requires action, watching, or nothing.
Questions to ask your doctor
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Written by: Cancer Explained Editorial TeamSources last checked: 2026-07-30Last updated: 2026-07-30Next planned review: 2027-07-30
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
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Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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