What is the approach to a solitary pulmonary nodule (SPN)?

Approach to Solitary Pulmonary Nodule

The management of a solitary pulmonary nodule (SPN) should follow a risk-stratified algorithm based on nodule size, morphology, and patient risk factors, with nodules ≥8 mm requiring formal risk assessment using validated prediction models (such as the Brock model), while smaller nodules (<5 mm) require no follow-up and nodules with benign calcification patterns can be dismissed without further evaluation. 1

Initial Characterization and Triage

Immediate Exclusion Criteria (No Follow-up Required)

• Nodules with benign calcification patterns (diffuse, central, laminated, or popcorn calcification) or macroscopic fat require no further investigation 1
• Typical perifissural or subpleural nodules (homogeneous, smooth, solid nodules with lentiform or triangular shape within 1 cm of a fissure or pleural surface and <10 mm) do not require follow-up 1
• Nodules <5 mm in diameter or <80 mm³ in volume do not require follow-up 1
• Solid nodules stable for ≥2 years on comparison imaging are very likely benign and require no further evaluation 2

Essential Imaging

• Perform thin-section CT (≤1 mm slices) through the nodule to characterize location, shape, margins, and attenuation characteristics 2
• Compare directly with all prior imaging (not just prior reports) to assess stability and growth 2

Risk Stratification Algorithm for Nodules ≥8 mm

Step 1: Calculate Malignancy Probability

Use the Brock model (full, with spiculation) for initial risk assessment, particularly in smokers or former smokers aged ≥50 1

Key clinical risk factors to incorporate:

• Increasing age 1
• Smoking history and pack-years 1
• Previous malignancy 1

Key radiological risk factors:

• Increasing nodule diameter 1
• Spiculation (odds ratio 2.2-2.5 for malignancy) 3
• Pleural indentation 1
• Upper lobe location 1, 3

Step 2: Management Based on Risk Category

Low Probability (<5-10% malignancy risk)

• CT surveillance is the recommended approach 1, 2
• Surveillance intervals for solid nodules 5-8 mm:
  • High-risk patients: CT at 6-12 months, then 18-24 months 1
  • Low-risk patients: Optional 12-month follow-up 1
• For nodules ≥6 mm: CT at 3 months, then reassess at 1 year 1
• Discontinue surveillance if no growth after 2 years 2

Intermediate Probability (5-10% to 60-70% malignancy risk)

• PET-CT is the next step for further risk stratification 1, 2
• PET-CT performance: Approximately 97% sensitivity and 78% specificity for nodules ≥1 cm 1
• After PET-CT results:
  • Negative or mild hypermetabolic: Consider nonsurgical biopsy or continued surveillance 2
  • Positive PET (high uptake): Proceed to tissue diagnosis or surgical resection 3

Important caveat: In Asian populations with high prevalence of granulomatous disease (tuberculosis, fungal infections), PET-CT has lower specificity due to false-positives from infectious/inflammatory conditions, and greater reliance on non-surgical biopsy is recommended 2, 1

High Probability (>60-70% malignancy risk)

• Proceed directly to tissue diagnosis or surgical resection 1, 2
• Pretreatment pathological diagnosis is strongly recommended before any curative treatment, obtained via biopsy of visible lesions 2
• Exception to biopsy requirement: An experienced multidisciplinary team may proceed without biopsy if the predicted likelihood of malignancy is ≥85% and the risk-benefit ratio of biopsy is unacceptable 2

Diagnostic Procedures

Biopsy Techniques Based on Nodule Location

For centrally located tumors:

• Bronchoscopy is the appropriate test, achieving pathological diagnosis in ~90% of cases via forceps biopsy, bronchial brushing, or washing 2

For peripheral lesions ≥8 mm:

• Percutaneous CT-guided biopsy is usually appropriate when results will alter management 1
• Advanced bronchoscopic techniques (radial EBUS or navigational CT-guided bronchoscopy) 2
• Video-assisted thoracoscopic surgery (VATS) biopsy for diagnostic excision 2

Biopsy considerations:

• Pneumothorax risk with transthoracic needle biopsy: 9-54%, higher in patients with underlying pulmonary disease 3
• Negative predictive value of biopsy is most useful when pretest probability is already low 2
• "Nonspecific benign" results require surgical confirmation in patients with adequate physiologic reserve, as 68% may still be malignant 4

Growth Assessment During Surveillance

Volume doubling time (VDT) interpretation:

• VDT <400 days: Indicates growth; requires further workup with PET-CT, biopsy, or resection 1
• VDT 400-600 days: Warrants continued surveillance or consideration of biopsy 1
• Volumetric analysis is preferred over diameter measurements when available for more accurate growth detection 1

Special Considerations for Subsolid Nodules

Part-solid nodules ≤8 mm:

• CT surveillance at 3,12, and 24 months, followed by annual CT for 1-3 additional years 1

Part-solid nodules >8 mm:

• Repeat CT at 3 months, then proceed to PET, nonsurgical biopsy, and/or surgical resection for persistent nodules 1

Critical Pitfalls to Avoid

• Do not rely solely on nodule size without considering morphology (spiculation, upper lobe location); this leads to underestimation of malignancy risk 3
• Do not delay evaluation of highly suspicious features (spiculated nodules >8 mm) waiting for growth documentation, as this allows disease progression 3
• Do not accept "nonspecific benign" biopsy results as definitive in patients with adequate surgical reserve and intermediate-to-high pretest probability 4
• Do not use PET-CT for nodules <8 mm, as sensitivity is inadequate 1
• In Asian populations, do not over-rely on PET-CT due to high false-positive rates from endemic granulomatous diseases 2
• Always measure attenuation on non-edge-enhanced images, as sharpened CT images give erroneously high values 3