Lung-RADS 3 Meaning: Categories 2 to 4B and Fleischner Intervals

Lung-RADS 3 meaning, in short: the screening CT found a lung nodule that is probably benign but should be checked again with a low-dose CT in 6 months. On the American College of Radiology's Lung-RADS v2022 scale, category 3 usually describes a solid nodule of 6 to under 8 mm at the first scan, or a new one of 4 to under 6 mm. It counts as a positive screen in audit terms, not as a diagnosis, and a nodule that stays stable returns you to yearly screening.

What Lung-RADS is and what each category means

Lung-RADS is the reporting system the American College of Radiology (ACR) publishes for low-dose CT (LDCT) lung cancer screening, the yearly scan offered to people at higher risk, mainly current and former smokers. Each scan gets one category from 0 to 4, based on the most suspicious nodule, together with a recommended next step. Categories 1 and 2 count as a negative screen and categories 3 and 4 as a positive screen, and the ACR adds that a negative screen does not mean that a person does not have lung cancer.

The size thresholds below are for solid nodules, measured as the mean of the long and short axis, taken from the ACR v2022 table:

Category	Typical solid nodule	Recommended next step
2, benign	under 6 mm at baseline, or new and under 4 mm	screening LDCT in 12 months
3, probably benign	6 to under 8 mm at baseline, or new and 4 to under 6 mm	LDCT in 6 months
4A, suspicious	8 to under 15 mm at baseline, new and 6 to under 8 mm, or growing and under 8 mm	LDCT in 3 months; PET/CT may be considered if 8 mm or larger
4B, very suspicious	15 mm or larger at baseline, or new or growing and 8 mm or larger	diagnostic chest CT, PET/CT, tissue sampling or referral, depending on clinical evaluation

Part-solid and ground glass nodules have their own thresholds. At baseline, a part-solid nodule of 6 mm or more with a solid part under 6 mm is category 3, a solid part of 6 to under 8 mm makes it 4A and 8 mm or more makes it 4B; on a follow-up scan, a new or growing solid part under 4 mm makes it 4A and one of 4 mm or more makes it 4B. A pure ground glass nodule, a hazy spot that does not hide the vessels, stays category 2 below 30 mm and becomes category 3 at 30 mm or more at baseline or when new. Category 4X is used when a category 3 or 4 nodule has extra worrying features, such as spiculation or enlarged lymph nodes.

The ACR estimates how often each result occurs in a screening population: about 45 percent of screens are category 2, 9 percent category 3, 4 percent category 4A and 2 percent category 4B. Category 0 means incomplete, for example while earlier CT scans are being located for comparison.

Lung-RADS 3 meaning in practice: the 6-month scan and stepped follow-up

A Lung-RADS 3 report usually ends with a recommendation for a low-dose CT in 6 months, counted from the date of the current scan. Under version 2022, follow-up is stepped: if the category 3 nodule is stable or smaller at 6 months, it becomes category 2 and the next screening CT follows 12 months after that scan. A category 4A nodule that is stable or smaller at its 3-month scan steps down to category 3 and is then rechecked at 6 months.

Growth has a precise definition in Lung-RADS: an increase in mean diameter of more than 1.5 mm within a 12-month interval. A nodule that crosses into a higher size band is also reclassified, even if the change is smaller. A solid or part-solid nodule that grows slowly over several screening rounds, without ever passing 1.5 mm in one year, may be classified as 4B, because slow growth is itself a warning sign.

Lung-RADS or Fleischner: which guideline applies to your nodule

Many lung nodules appear on a CT done for chest pain, a cough or an injury. For these incidental nodules, radiologists use the Fleischner Society guidelines of 2017 (MacMahon, Radiology 2017), which apply to adults aged 35 and over and explicitly not to screening, to people with a known cancer or to people with a weakened immune system.

The Fleischner intervals are given as ranges so that the doctor and patient can weigh individual risk.

A single solid nodule under 6 mm needs no routine follow-up in a person at low risk. A solid nodule of 6 to 8 mm is rechecked at 6 to 12 months; at high risk, a second scan at 18 to 24 months is added. For a solid nodule larger than 8 mm, the guideline suggests considering a CT at 3 months, a PET/CT, tissue sampling or a combination. A pure ground glass nodule of 6 mm or more is followed at 6 to 12 months and then every 2 years until 5 years.

The two systems use similar size bands but different wording and rounding, so a report should name the system it used. A Lung-RADS category on an incidental nodule, or Fleischner intervals on a screening scan, is a reason to ask which pathway applies.

What the radiologist checks on a lung nodule CT

In our reading room, a lung nodule CT is read in a fixed order, because several decisions come before any number.

1. The reason for the scan. Screening CT or diagnostic CT, age, smoking history and any known cancer decide whether Lung-RADS, Fleischner or neither applies.
2. Image quality. Fleischner recommends thin sections of 1.5 mm or less, with coronal and sagittal reconstructions. Thick slices can make a part-solid nodule look like pure ground glass, or hide fat or calcium that would mark a nodule as benign.
3. The nodule type. Solid, part-solid or ground glass changes every threshold. This is the hardest step: Fleischner cites a study in which experienced thoracic radiologists all classified nodules correctly as solid or subsolid in only 58 percent of cases.
4. The measurement. Long and short axis on the image showing the largest size, averaged into a mean diameter. Lung-RADS reports it to one decimal place, Fleischner rounds to the nearest whole millimetre, so 5.6 mm can be "under 6 mm" in one system and "6 mm" in the other. For a part-solid nodule, the solid part is measured separately.
5. Benign features and position. Fat or a typical calcification pattern makes a nodule category 1 in Lung-RADS. Juxtapleural nodules (next to the pleura or a fissure) that are solid, smooth, oval, lentiform or triangular and under 10 mm in mean diameter are category 2; the Fleischner Society describes such triangular nodules as typical of a small lymph node within the lung.
6. The risk-dominant nodule. With several nodules, the most suspicious one, not always the largest, sets the category.
7. Comparison with every prior CT. Stability over earlier scans can lower a category; a new nodule raises it. Without the priors, the category is often provisional.
8. The conclusion. A good report states the system used, the nodule type, size and location, comparison, the category and the recommended interval.

How often radiologists disagree on a Lung-RADS category

In a European study, seven readers assigned Lung-RADS categories to 160 screening CTs from the US National Lung Screening Trial: agreement was substantial (mean kappa 0.67), yet the category differed in 29 percent of the 3'360 reading pairs, and the difference would have changed the follow-up plan in 8 percent (van Riel, European Radiology 2018). Most disagreements, 74 percent, arose because readers chose a different nodule as the risk-dominant one, not because they measured the same nodule differently.

A 2025 meta-analysis of 11 studies with 1'470 patients found a pooled kappa of 0.72 for Lung-RADS categories, falling to 0.66 in studies with manual diameter measurement and to 0.55 in studies where fewer than 30 percent of nodules were solid (Hwang, American Journal of Roentgenology 2025). The limits matter: the van Riel sample was enriched to hold equal numbers of each category, the scans came from a US trial and were read under research conditions with an earlier Lung-RADS version, so the figures describe how hard the task is, not an error rate for any single report. In the van Riel study, of 91 reading pairs on scans with a cancer diagnosed within a year, only two disagreements would have led to a substantially different plan.

What a second read can change about a lung nodule report

A second read can change how a nodule is classified and what happens next.

• If the category is confirmed, the follow-up interval stands.
• If it is lowered, for example a 4A nodule that a direct comparison shows has been stable for years, an early scan or a PET/CT may no longer be needed.
• If it is raised, for example when a ground glass nodule turns out to have a solid part on thin sections, the next step comes sooner.
• If a different guideline fits the scan, the report can be restated in that system.

Any decision about further scans, biopsy or referral is made with your treating physician or the screening programme. If you cough up blood, become suddenly short of breath or have new chest pain, seek urgent medical care rather than waiting for a report.

When a second opinion on Lung-RADS 3 or 4A is worth it

A second read is most useful when:

• earlier chest CTs exist but were not compared;
• the nodule sits near a size threshold, or is part-solid, where agreement between readers is lowest;
• the report is 4A or 4B and a PET/CT, biopsy or operation is being planned;
• the report does not say which system it used, or the nodule type and measurement are missing;
• several nodules are present and it is unclear which one sets the category.

It is less likely to help when a small solid nodule has already been stable on follow-up and your doctor has discussed the plan with you, or when the next scan in a screening programme is only weeks away and will answer the question.

Lung screening in Switzerland and Europe, and how to send your images

In Switzerland, the Swiss Cancer Screening Committee issued a conditional recommendation in favour of opportunistic low-dose CT lung screening in November 2022, and regional pilot studies have been run, among them one at the University Hospital of Zurich, which began enrolling in 2018, and a community-based study in Ticino. In the Ticino pilot of 100 people at risk, 10 percent had a category 3 result and 4 percent a category 4A result, and three lung cancers were found at the first scan or at follow-up (Patella, BMC Pulmonary Medicine 2026). In Europe, the European Society of Radiology announced on 2 November 2026 that SOLACE+, a three-year project with 46 partners from 22 EU Member States and Norway, will support 19 countries in setting up screening; the original SOLACE project screened more than 30'000 people in 11 countries.

For a second read, ask the imaging centre for the complete CT as DICOM files, including the thin-section series, together with the report and every earlier chest CT. Our guide on how to read your radiology report explains the sections you will see, and the article on the Fazekas scale shows how another imaging score is read. The CT second opinion page lists what is included.

Alpine Diagnostics is a Swiss practice in Zürich whose board-certified subspecialist radiologists provide second opinions for patients and teleradiology for institutions. A named radiologist signs every report, every question you submit is answered in writing, in English, German, Russian or Italian, and standard delivery is within 72 hours, with Priority delivery within 24 hours as an option.

If a biopsy, PET/CT or operation depends on whether your nodule is category 3, 4A or 4B, two radiologists reading the CT independently and agreeing one report gives you a checked category and interval.