A low-dose CT scan is a type of computed tomography scan that uses significantly less radiation than a standard CT to produce detailed images of the lungs. It’s most commonly used to screen for lung cancer in people with a history of smoking, and it’s the only screening test proven to reduce lung cancer deaths. In clinical trials, annual low-dose CT screening lowered lung cancer mortality by 20% compared to traditional chest X-rays.
How It Differs From a Standard CT
Both standard and low-dose CT scans work the same way: an X-ray tube rotates around your body, capturing cross-sectional images that a computer assembles into detailed pictures. The key difference is the amount of radiation. A standard chest CT delivers roughly 1.8 millisieverts (mSv) of radiation on average, while a low-dose scan can bring that down to about 0.3 mSv, an 83% reduction. For context, 0.3 mSv is in the range of just a few chest X-rays.
The scanner achieves this by lowering the electrical current flowing through the X-ray tube. Modern CT systems also use automatic exposure control, which adjusts the X-ray intensity in real time based on your body’s thickness and density at each angle. As the tube rotates, it increases the dose through thicker tissue (like shoulders) and reduces it through thinner areas (like the abdomen), cutting unnecessary radiation by 20 to 40% on its own. The tradeoff is slightly grainier images, but for lung screening, the contrast between air-filled lung tissue and solid nodules is high enough that the image quality remains diagnostic.
What Happens During the Scan
The entire procedure takes only a few minutes. You lie on a flat table that slides into a doughnut-shaped scanner. There’s no injection of contrast dye, and in most cases no fasting or special preparation is needed. You’ll be asked to hold your breath for about 11 seconds while the scanner captures images of your chest. That single breath-hold is usually enough to image the entire lung field. You can return to normal activities immediately afterward.
Who Should Get Screened
Low-dose CT screening is specifically recommended for people at high risk of lung cancer due to smoking. The U.S. Preventive Services Task Force recommends annual screening for adults aged 50 to 80 who have a smoking history of at least 20 pack-years and either currently smoke or quit within the past 15 years. A pack-year means averaging one pack of cigarettes per day for one year, so someone who smoked two packs a day for 10 years would have a 20 pack-year history.
The American Cancer Society updated its guidelines in 2023 with one notable change: for people who formerly smoked, the number of years since quitting is no longer a factor in eligibility. Under ACS guidelines, if you’re 50 to 80 with a 20 pack-year history, you qualify for annual screening regardless of when you stopped smoking. The USPSTF still uses the 15-year quit cutoff, so which criteria apply to you may depend on your insurance plan and your doctor’s recommendation.
Screening should stop once a person reaches 80, has not smoked for 15 years (under USPSTF guidelines), or develops a health condition that would make lung surgery impractical.
What the Results Mean
Radiologists use a standardized scoring system called Lung-RADS to categorize findings. Understanding the categories can help you make sense of your results:
- Category 1 (Negative): No lung nodules found. You continue with annual screening.
- Category 2 (Benign): Small nodules were spotted, but their size or stability over time suggests they’re not cancerous. Annual screening continues as usual.
- Category 3 (Probably benign): A nodule was found that has a low likelihood of being cancer but warrants a closer look. You’ll typically be asked to come back for a follow-up scan in six months.
- Category 4A (Suspicious): A nodule is large enough or has changed enough to need further evaluation, usually with another scan in three months. In some cases, a PET scan may be ordered.
- Category 4B (Very suspicious): Findings strongly suggest the possibility of cancer. Additional imaging or a tissue biopsy is typically the next step.
Most people who get screened will fall into categories 1 or 2, and the vast majority of nodules found on low-dose CT are not cancer.
The False Positive Problem
One of the most important things to understand about low-dose CT screening is the high rate of false positives. In the National Lung Screening Trial, roughly 24 to 27% of initial screenings flagged something that ultimately turned out not to be cancer. By the third round of screening, that rate dropped to about 16%, partly because radiologists could compare new images to prior ones and confirm that stable nodules hadn’t changed.
A false positive doesn’t necessarily mean an invasive procedure. Most of the time, it leads to a follow-up CT scan in three to six months to check whether a nodule has grown. When stricter size thresholds are used (flagging only nodules 9 mm or larger instead of 5 mm), the false positive rate drops to around 3.4%. Screening programs have refined their protocols over time to reduce unnecessary follow-up testing, but the possibility of a scare that turns out to be nothing remains one of the real downsides of screening.
Coverage and Cost
Medicare Part B covers one low-dose CT lung cancer screening per year with no out-of-pocket cost, as long as your provider accepts Medicare’s standard payment rates. Medicare’s eligibility criteria are slightly different from the USPSTF’s: coverage applies to adults aged 50 to 77 (not 80) with at least a 20 pack-year smoking history who currently smoke or quit within the past 15 years. You need to be symptom-free and have a doctor’s order. Before your first screening, Medicare requires a shared decision-making visit with your provider to discuss the benefits and risks.
Most private insurance plans also cover lung cancer screening without cost-sharing for people who meet the USPSTF criteria, because the Affordable Care Act requires coverage of services that receive a “B” recommendation from the Task Force. If you don’t meet the eligibility criteria, the scan would not be covered as a preventive service, and you could face significant out-of-pocket costs.
Benefits and Tradeoffs
The clearest benefit of low-dose CT screening is catching lung cancer early, when it’s most treatable. Lung cancer is the leading cause of cancer death in the United States, and most cases are diagnosed at an advanced stage. The 20% mortality reduction seen in clinical trials translates to real lives saved, particularly among people with heavy smoking histories.
The tradeoffs are worth weighing honestly. False positives are common, especially in the first year of screening, and can cause anxiety and lead to additional scans or, less commonly, biopsies that carry their own small risks. There’s also a small cumulative radiation exposure from annual scans, though at 0.3 mSv per scan, this is quite low. And screening occasionally detects slow-growing cancers that might never have caused symptoms during a person’s lifetime, a phenomenon called overdiagnosis, which can lead to treatment that wasn’t truly necessary.
For people who meet the eligibility criteria, the evidence strongly favors screening. The mortality benefit outweighs the risks for this high-risk group. For people outside the criteria, the balance shifts, which is why screening isn’t recommended for the general population.