ACR Lung-RADS Overview
The Lung-RADS (Lung CT Screening Reporting and Data System) is the American College of Radiology's standardized framework for reporting and managing pulmonary nodules detected on low-dose CT lung cancer screening. Published in 2015 and updated to v1.1 in 2019, Lung-RADS assigns each LDCT screening exam one of six categories — 1, 2, 3, 4A, 4B, or 4X — and ties each to a recommended LDCT follow-up interval and PET/CT / biopsy consideration. For incidental (non-screening) pulmonary nodules, the Fleischner Society 2017 guidelines apply instead. Like TI-RADS for thyroid and BI-RADS for breast, Lung-RADS provides a standardized vocabulary and decision pathway that aligns radiologists, pulmonologists, and oncologists on next steps.
How to Use This Calculator
Score the nodule across five input groups to receive a Lung-RADS category:
- Nodule Type — Select the predominant morphology on the LDCT: solid, part-solid (mixed solid + ground-glass), or pure non-solid / ground-glass nodule (GGN). Type drives which size thresholds apply.
- Nodule / Total Lesion Size — Enter the largest axial dimension in millimeters of the entire nodule or lesion (including any surrounding GGN component for part-solid nodules).
- Solid Component Size — Only for part-solid nodules: enter the size of the solid component (between 0 mm and the total lesion size). Determines LR3 vs LR4A vs LR4B sub-classification.
- New on Follow-up — Check if this nodule was not present on a prior LDCT. New solid nodules carry tighter size thresholds (LR2 if <4 mm, LR3 if 4–5 mm, LR4A if ≥6 mm at baseline follow-up).
- Modifier 4X Features — Check any that apply: spiculation, concurrent ground-glass nodule with solid component, growth on follow-up, lymphadenopathy, endobronchial nodule, pleural effusion, or metastases. Any one of these on a baseline LR3-LR4B nodule upgrades the exam to 4X.
Lung-RADS Categories and Management
| Category | Solid (mm) | Part-solid (mm) | GGN (mm) | Management |
|---|---|---|---|---|
| LR1 — Negative | None | None | None | Continue annual LDCT |
| LR2 — Benign-appearing | <6 | <6 total | <20 | Continue annual LDCT |
| LR3 — Probably benign | 6–7 | ≥6 total, solid <6 | ≥20 | 6-month follow-up LDCT |
| LR4A — Suspicious | 8–14 | solid 6–7 | — | 3-month LDCT; consider PET/CT |
| LR4B — Very suspicious | ≥15 | solid ≥8 | — | 3-month LDCT; PET/CT + tissue sampling |
| LR4X — 4 + other findings | Any LR3 / LR4 with spiculation, growth, GGN-with-solid, lymphadenopathy, endobronchial nodule, effusion, or metastases | 3-month LDCT; PET/CT + tissue sampling | ||
New Nodules on Follow-up LDCT
New solid nodules identified on annual follow-up LDCT carry tighter size thresholds than baseline nodules:
- New solid <4 mm: LR2 — return to annual screening.
- New solid 4–5 mm: LR3 — 6-month follow-up LDCT.
- New solid ≥6 mm: LR4A — 3-month LDCT with PET/CT consideration.
New part-solid and new pure GGN nodules are categorized by the same size criteria as baseline nodules, but warrant extra attention because new GGNs can represent indolent adenocarcinoma spectrum lesions.
Modifier 4X — When Categories Aren't Enough
Category 4X is reserved for size-defined LR3 or LR4 nodules that have additional imaging features raising concern beyond size alone. Modifier 4X features include:
- Spiculation — radiating spiculated margins represent desmoplastic reaction characteristic of invasive malignancy.
- GGN with solid component — concurrent subsolid morphology on this or another lobe raises concern for multifocal adenocarcinoma.
- Growth on follow-up LDCT — any new or enlarging component over serial LDCT.
- Lymphadenopathy — hilar or mediastinal nodes ≥10 mm short axis, particularly if new.
- Endobronchial nodule — lesions within an airway lumen.
- Pleural effusion — without other explanation.
- Rib or extra-thoracic disease — suggestive of metastatic spread.
Any one Modifier 4X feature on an LR3, LR4A, or LR4B nodule upgrades the exam to 4X. Management mirrors LR4B: 3-month LDCT, PET/CT, consideration of tissue sampling, and multidisciplinary nodule conference discussion when available.
Interpreting Your Lung-RADS Result
A Lung-RADS category drives LDCT follow-up intervals and PET/CT decisions. Key interpretation principles:
- LR1 and LR2: Continue annual LDCT. LR1 represents no nodules or only nodules with clearly benign features (popcorn, complete central, or diffuse calcification). LR2 covers perifissural, sub-solid, or small solid nodules below category thresholds. Positivity rate (Lung-RADS 3 or 4) on a baseline screen should be ≤10%; higher rates suggest over-reading.
- LR3 (probably benign): 6-month follow-up LDCT. Estimated malignancy prevalence <1–2%. If stable at 6 months, return to annual LDCT. Interval growth or new features upgrade to LR4A, LR4B, or LR4X.
- LR4A (suspicious): 3-month follow-up LDCT; PET/CT considered. Malignancy prevalence roughly 5–15%. PET avidity (SUV > baseline mediastinal uptake) raises concern and may tip the discussion toward biopsy.
- LR4B (very suspicious): 3-month follow-up LDCT with PET/CT and tissue sampling considered. Malignancy prevalence >15%. Biopsy or surgical resection is generally warranted for surgical candidates; for non-surgical candidates, surveillance with PET/CT is reasonable.
- LR4X: 3-month LDCT with PET/CT and tissue sampling. The Modifier features (spiculation, growth, lymphadenopathy, etc.) place these nodules in a higher risk band than size alone. Routine referral to thoracic surgery or multidisciplinary review is appropriate.
Limitations & Considerations
Lung-RADS v1.1 has important limitations clinicians should be aware of:
- Inter-observer variability: Morphology (solid vs part-solid vs GGN) and measurement precision on LDCT have meaningful inter-reader variation. Category assignment can shift between readers on the same nodule; correlations with prior imaging and real-time measurement tools reduce but do not eliminate this.
- Limited out-of-screening validation: Lung-RADS categories are derived for asymptomatic adults meeting USPSTF lung cancer screening criteria (age 50–80, ≥20 pack-years, current smoker or quit within 15 years). The categories are not directly validated for incidentally discovered nodules — apply Fleischner 2017 in that setting.
- Does not replace clinical judgment: Family history of lung cancer, prior cancer, second-hand smoke or occupational exposure, and patient-specific surgical candidacy may justify workup below ACR category thresholds. Lung-RADS guides — it does not dictate.
- Measurement precision: Small differences in size (1–2 mm) cross category thresholds. Use consistent measurement technique (axial largest dimension for solid, both solid component and total lesion for part-solid) and corroborate with prior exams when available.
- Lung-RADS v2022 pending rollout: ACR released a v2022 update with refined categories (LR1, LR2, LR3, LR4A, LR4B, LR4X preserved but category modifications) — this calculator implements v1.1 for broadest validation and clinical familiarity. Subsolid nodule pathway (now a parallel "S" modifier for confirmed GGNs) will become more important in future iterations.