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This is a shared Orris chat about the Radiological Approach to the Solitary Pulmonary Nodule (based on Grainger & Allison's textbook). Here's the full content from that conversation:

Radiological Approach to the Solitary Pulmonary Nodule

(Grainger & Allison)

Definition

A solitary pulmonary nodule (SPN) is a single, well-defined, rounded opacity ≤3 cm in diameter, surrounded by lung parenchyma, with no associated lymphadenopathy or atelectasis.

Clinical Significance

SPNs are common incidental findings on chest CT. The key clinical question is distinguishing benign from malignant lesions, as ~40% of SPNs in high-risk populations may be malignant.

Step 1: Assess Nodule Size and Growth Rate

Size	Interpretation
<5 mm (<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

CT volumetry of a right upper lobe nodule showing automatic segmentation (A), volume calculation (B), and 49% volume increase at 1-year follow-up (C)

Step 2: Assess Location, Shape, and Morphology

Perifissural Nodules (Benign Features)

Nodules attached to or within 1 cm of a fissure, with a polygonal, lenticular, or triangular shape and smooth margins - these are almost always benign intrapulmonary lymph nodes and can be discharged.

Nodule Outline

Spiculated/corona radiata: Highly suspicious for malignancy
Lobulated: Intermediate suspicion
Smooth: More likely benign, but not excluded

CT showing multiple solid nodules (A - benign calcified nodules/granulomas) and a spiculated nodule with corona radiata (B - highly suspicious for malignancy)

Step 3: Assess Calcification Pattern

CT showing a mildly lobulated calcified nodule in the left lower lobe consistent with hamartoma (popcorn calcification pattern)

Calcification patterns:

Benign: Central, diffuse, laminated ("eggshell"), or popcorn (hamartoma)
Indeterminate/malignant: Eccentric, amorphous, or stippled calcification

Step 4: Classify as Solid vs. Subsolid

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

Part-solid nodules with a solid component >5 mm carry the highest malignancy risk of all SPN subtypes.

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 mm³ (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)

A validated malignancy prediction tool incorporating: family history of lung cancer, emphysema on CT, nodule size, spiculation, upper lobe location, nodule type (part-solid > solid > GGN), and count.

Step 6: Advanced Imaging Modalities

FDG-PET / PET-CT

Best for nodules >8 mm
Sensitivity ~87%, specificity ~83% for malignancy
False negatives: carcinoid, BAC/AIS, mucinous adenocarcinoma, small nodules
False positives: active infection/inflammation (TB, fungal, rheumatoid nodule)

CT Enhancement

Benign: <15 HU enhancement
Malignant: >15 HU enhancement (sensitivity ~98%, but low specificity)

MRI

Emerging role; DWI useful for characterizing nodules, avoids radiation

Step 7: Tissue Sampling

CT-guided percutaneous biopsy: For peripheral nodules >8 mm; risk of pneumothorax ~15-20%
Bronchoscopy/EBUS: For central or peribronchial nodules; radial EBUS + fluoroscopy improves yield for peripheral lesions
VATS: Diagnostic and potentially curative for peripheral nodules; preferred if high malignancy suspicion

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 >5 mm)
VDT	>600 days or stable >2 years	<400 days
FDG-PET	No uptake	Uptake > mediastinal blood pool
CT enhancement	<15 HU	>15 HU

That's the full shared chat. It covers the complete 7-step radiological approach to SPNs from Grainger & Allison. Would you like me to expand on any particular step, compare additional guidelines (e.g. Lung-RADS, ACCP), or discuss a specific clinical scenario?

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