Solitary Pulmonary Nodule: Definition, DDx, and Evaluation with CT

Axial scan of the lung nodule in mediastinal window reveals a solid component of the nodule without calcification

Axial scan of the lung nodule in lung window shows a lobulated, smooth margin of the nodule

Definition of SPN

• Radiographic opacity of equal to or less than 3 cm
• At least two thirds of margins are surrounded by lung parenchyma
• Exclusion of lymph nodes (not always possible), atelectasis and postobstructive pneumonia

Differential Diagnosis

• Infectious: TB, pneumonia, abscess, fungus, etc
• Neoplastic: benign and malignant
• Vascular: AVM, infarct, aneurysm venous varix, hematoma
• Congenital: bronchogenic cyst, sequestration, bronchial atresia
• Inflammatory: rheumatoid arthritis, Wegener, sarcoidosis, microscopic angiitis
• Lymphatic: lymph node, lymphoma
• Outside the lungs: skin nodule, nipple shadows, rib fracture, pleural lesion
• Miscellaneous: rounded atelectasis, lipoid pneumonia, amyloidosis, etc

CT Evaluation

• Thin-section (1 mm) contiguous images through nodule are suggested
• Both lung and mediastinal windows obtained (lung window for margins/edges, mediastinal window for solid component)
• Low dose (less than 80 mAs) can be used for purpose of nodule characterization

CT Characteristics

• Growth rate: malignancy likely if doubling time = 20-400 days
• Size: malignancy likely if size > 3 cm
• Margin/border/edge: malignancy likely if lobulated, speculated, ragged, halo, notches
• Calcification: attenuation > 200 HU indicates presence of calcium in the nodule. No pattern of calcification specific for malignancy
• Cavitation: malignancy likely if irregular and thick (> 15 mm) wall

Our case: Solitary pulmonary nodule from a metastatic colon cancer

Reference:

Patel VK, Naik SK, Naidich DP, et al. A practical algorithm approach to the diagnosis and management of solitary pulmonary nodules. Part 1: radiologic characteristics and imaging modalities