What is a Lung Nodule on Chest CT?
A lung nodule is a well or poorly circumscribed, approximately rounded focal opacity that appears on chest CT imaging, measuring ≤3 cm in diameter and surrounded by aerated lung, which may be single or multiple and exists without associated abnormalities like lymphadenopathy or pleural disease 1.
Key Defining Characteristics
Lung nodules are categorized by their size and attenuation pattern on CT imaging:
Size Classification
- Small solid nodules: <8 mm in diameter
- Larger solid nodules: ≥8 mm in diameter
- Subsolid nodules: Further subdivided based on composition 2
Attenuation Types
Solid nodules appear as completely opaque structures that obscure underlying lung parenchyma.
Subsolid nodules are divided into two critical subtypes 3, 1:
- Pure ground-glass nodules (pGGN): Hazy opacities with no solid component, through which underlying vessels remain visible
- Part-solid nodules: Contain both ground-glass and solid components
Clinical Significance
At least 95% of all pulmonary nodules are benign, most commonly representing granulomas or intrapulmonary lymph nodes 2. However, the detection rate is substantial—nodules appear on approximately 30% of chest CT scans and affect roughly 1.6 million patients annually in the US 2.
Malignancy Risk Stratification
The probability of malignancy correlates directly with nodule size:
- <6 mm: <1% malignancy risk
- 6-8 mm: 1-2% malignancy risk
- ≥8 mm solid: Higher risk requiring individualized assessment
- Part-solid nodules with solid component ≥6 mm: Should be considered highly suspicious 3
- Persistent ground-glass nodules >10 mm: 10-50% malignancy probability 2
Technical Detection Requirements
All chest CT scans in adults must be reconstructed with contiguous thin sections (≤1.5 mm, typically 1.0 mm) to enable accurate nodule characterization 3. Thick sections increase volume averaging and preclude accurate assessment of part-solid morphology, fat content, or calcium—all critical factors affecting management decisions.
Common Pitfall
Using thick CT sections can cause a part-solid nodule to appear as a pure ground-glass nodule, potentially leading to inappropriate surveillance intervals rather than more aggressive management 3.
Benign Features That Require No Follow-up
Certain CT characteristics indicate definitively benign nodules 3:
- Smoothly marginated nodules with internal fat and calcification (hamartoma)
- Nodules with central or laminar calcification patterns (healed granuloma)
These findings allow immediate cessation of surveillance without additional imaging.