Radiological Approach to solitary pulmonary nodule by garagier and allison
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Radiological Approach to the Solitary Pulmonary Nodule
Source: Grainger & Allison's Diagnostic Radiology (7th ed.), Chapter 8 - Lung Cancer and Pulmonary Nodules
Definition
A solitary pulmonary nodule (SPN) is a single, discrete, rounded opacity in the lung parenchyma that measures up to 3 cm in diameter and is not associated with mediastinal adenopathy, atelectasis, or pleural effusion. Lesions >3 cm are classified as masses and carry a higher probability of malignancy.
Clinical Significance
Nodule detection has become an everyday occurrence with multislice CT. Management presents three possible outcomes:
- No further action required
- CT surveillance
- Further investigation (PET/CT and/or biopsy)
Step 1: Assess Nodule Size and Growth Rate
Nodule size is the dominant parameter guiding management.
| Size | Interpretation |
|---|---|
| <5 mm (or <80 mm³) | Malignancy risk so low that follow-up not generally recommended in average-risk patients |
| 5-8 mm | Intermediate risk - CT surveillance guided by volumetry/VDT |
| >8 mm | Significant risk - further assessment with PET/CT or tissue sampling |
Volume Doubling Time (VDT): The BTS guidelines advocate volumetry to calculate VDT at follow-up. Growing nodules with a VDT <600 days are significantly more likely to be malignant. Some indolent adenocarcinomas (usually subsolid) have considerably prolonged VDTs.
Step 2: Assess Location, Shape, and Morphology
Perifissural Nodules (Benign Features)
These small subpleural nodules frequently represent intrapulmonary lymphoid tissue or intraparenchymal lymph nodes. Characteristic features:
- <15 mm from the pleural surface
- Ellipsoid shape
- Connected to pleura by a fine linear opacity
- Lower lobe predominance
- Follow-up studies from the NELSON Screening Trial confirmed no perifissural nodule developed into lung cancer
Nodule Outline
- Concave surfaces on all sides, or a straight surface of contact with the pleura - benign features
- The less spherical a nodule (low sphericity index on volumetric CT), the less likely a malignant aetiology
- Flat or tubular nodules are more likely to be benign
- Solid, subpleural, polygonal nodules with low sphericity = highly unlikely to be malignant
- Spiculation ("corona radiata") - irregular spiculated margins are associated with malignancy
Step 3: Assess Calcification Pattern
Calcification pattern is one of the most reliable CT features for benign versus malignant assessment.
Benign Patterns (specific enough to preclude further evaluation):
- Central (bull's eye) - granuloma
- Diffuse/homogeneous - old granuloma
- Laminated/concentric rings - granuloma
- Popcorn calcification - hamartoma (cartilaginous nodule)
Malignant / Indeterminate Patterns:
- Eccentric / amorphous calcification within soft-tissue density - may be seen in malignancy (dystrophic calcification within a tumour)
- Note: metastases from bone-forming or cartilage-forming tumours may occasionally show benign-appearing calcification
Step 4: Classify as Solid vs. Subsolid
Subsolid Nodules
| Type | Definition | CT Appearance | Histological Correlate |
|---|---|---|---|
| Pure Ground-Glass Nodule (GGN) | Focal increased attenuation, lung architecture visible | Hazy, no solid component | Atypical adenomatous hyperplasia, adenocarcinoma in situ |
| Part-Solid Nodule | GGN with a central solid component that obscures architecture | Mixed density | Minimally invasive adenocarcinoma, invasive adenocarcinoma |
The Fleischner Society and BTS have published dedicated guidelines for subsolid nodules, recognising they may represent early forms of adenocarcinoma.
New adenocarcinoma classification (replacing bronchoalveolar carcinoma):
- Pre-malignant: Atypical adenomatous hyperplasia + adenocarcinoma in situ - appear as pure GGN, ≤1 cm, pure lepidic growth, no solid components. Small GGNs (<5-6 mm) do not require surveillance.
- Minimally invasive adenocarcinoma (MIA): Predominantly lepidic growth, ≤3 cm, invasive component ≤5 mm, generally subsolid with solid component usually <5 mm. May grow slowly.
- Invasive adenocarcinoma: Solid component >5 mm.
The solid component of a part-solid nodule is the invasive portion and drives management. A part-solid nodule that started as a pure GGN and develops a solid centre (Fig. 8.5B) requires prompt evaluation.
Step 5: Risk Stratification and Management Algorithms
Fleischner Society 2017 vs. BTS 2015 Guidelines
| Parameter | Fleischner 2017 | BTS 2015 |
|---|---|---|
| Population | Age ≥35, incidentally detected | Age ≥18, all presentations |
| Sizing method | Mean diameter (long + short axis) | Volumetry preferred |
| Follow-up threshold | 6 mm | 80 mm² (or 5 mm) |
| Growth definition | ≥2 mm diameter increase | >25% volume increase |
| Small nodule (<6 mm) | No follow-up in low risk | Discharge; follow if definitely new vs. prior 1-2y CT |
| Medium nodule (6-8 mm) | CT at 6-12 months, then 18-24 months | CT at 3 months; further follow-up based on VDT |
| Large nodule (>8 mm) | CT at 3 months, PET/CT or biopsy | Brock model risk estimation |
BTS Brock Model (>8 mm nodules)
- Brock model risk <10%: CT surveillance
- Brock model risk 10-70%: Consider image-guided biopsy, excision, or surveillance; further risk stratification with Herder model (incorporating FDG-PET result)
- Herder score <10%: CT surveillance
- Herder score 10-70%: Image-guided biopsy, excision, or surveillance
- Herder score >70%: Consider definitive treatment (resection)
Step 6: Advanced Imaging Modalities
FDG-PET / PET-CT
- Indicated for solid nodules >8 mm that cannot be resolved by CT morphology alone
- Sensitivity: 82-97%, Specificity: 78-82% for malignancy
- False positives: Active granulomas (TB, sarcoidosis, aspergillosis)
- False negatives: Hyperglycaemia; low metabolic tumours (carcinoid, BAC/lepidic adenocarcinoma); nodules <8 mm in diameter
- Integrated PET/CT provides anatomical and functional assessment
CT Enhancement
- Nodule enhancement >15 HU after contrast suggests increased vascularity (malignancy)
- Limited specificity; largely superseded by PET-CT
MRI
- Limited role in nodule characterisation; useful in selected cases
Step 7: Tissue Sampling
Once imaging favours malignancy, tissue diagnosis is required:
- CT-guided percutaneous biopsy - for peripherally located nodules accessible to needle
- Bronchoscopy / EBUS - for centrally located or hilar lesions
- VATS / surgical resection - if non-diagnostic biopsy or high surgical fitness and high malignancy probability
Summary: Radiological Features Favouring Benign vs. Malignant
| Feature | Favours Benign | Favours Malignant |
|---|---|---|
| Size | <5 mm | >8 mm |
| Shape | Polygonal, flat, tubular, low sphericity | Round, spherical |
| Margins | Smooth, well-defined, concave surfaces | Spiculated (corona radiata), lobulated |
| Location | Subpleural, perifissural, lower lobe | Upper lobe (especially right) |
| Calcification | Central, diffuse, laminated, popcorn | Absent, eccentric, amorphous |
| Density | Solid with fat density (hamartoma) | Part-solid (solid component >5mm) |
| VDT | >600 days or stable >2 years | <400 days |
| FDG-PET | No uptake | Uptake > mediastinal blood pool |
| CT enhancement | <15 HU | >15 HU |
Source: Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging (ISBN 9780323760751), Chapter 8, pp. 184-208. Supplementary content from Harrison's Principles of Internal Medicine and pulmonology references.
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