Can a solid lung nodule found on a computed tomography (CT) chest scan originate from a previous lung infection?

Solid Lung Nodules on CT Chest and Their Relationship to Previous Lung Infections

Yes, solid lung nodules found on CT chest scans can originate from previous lung infections. 1 These nodules often represent healed granulomata from prior infections, especially in regions with endemic fungal infections.

Characteristics of Post-Infectious Nodules

• Solid nodules in the size range of 5-8mm are frequently encountered in routine clinical practice and are usually benign in origin, most often representing either healed granulomata from a previous infection or intrapulmonary lymph nodes 1
• Small nodules that are stable for at least 2 years are typically considered benign 1
• Triangular subpleural nodules with linear extension to the pleural surface are characteristic of intrapulmonary lymph nodes and do not require follow-up 1
• Nodules with diffuse, central, laminated, or "popcorn" calcification patterns are considered benign and often represent healed granulomatous disease 1, 2

Differentiating Features Between Benign and Malignant Nodules

Features Suggesting Benign Etiology (Including Post-Infectious)

• Smooth or polygonal margins (likelihood ratio of 0.2 for malignancy) 1, 2
• Combination of round shape, smooth margins, and low attenuation 1, 2
• Peripheral location 2
• Presence of specific calcification patterns (diffuse, central, laminated, or popcorn) 1, 2
• Stability in size for at least 2 years 1

Features Suggesting Malignancy

• Spiculated or irregular margins (likelihood ratio of 5.5 for malignancy) 1, 2
• Upper lobe location 2
• Pleural retraction (likelihood ratio of 1.9 for malignancy) 1, 2
• Vessel sign (likelihood ratio of 1.7 for malignancy) 1, 2
• Growth on serial imaging with volume doubling time less than 400 days 2
• Size greater than 8mm 2

Management Considerations

• For solid nodules smaller than 6mm (5mm or less) in diameter, no routine follow-up is recommended due to very low risk of malignancy 1
• For nodules 6-8mm, follow-up CT at 6-12 months may be considered, especially in high-risk patients 1, 3
• In patients with clinical evidence of infection and in immunocompromised patients, active infection should be considered, and short-term follow-up may be appropriate 1
• The same technical parameters should be used for each screening CT to ensure accurate size measurements 1

Special Considerations

• When multiple nodules are present, the risk of primary cancer increases as the total nodule count increases from 1 to 4, but decreases for those with five or more nodules, which likely result from prior granulomatous infection 1
• In regions with endemic fungal infections, benign post-infectious nodules are more common 1
• Patient risk factors (smoking history, age, prior malignancy) should be incorporated into the assessment of nodule risk 3, 2

Pitfalls to Avoid

• Don't assume all solid nodules are malignant - many are benign post-infectious changes 1, 2
• Don't overlook the importance of comparing with prior imaging to establish stability 1
• Avoid unnecessary follow-up for clearly benign nodules with characteristic calcification patterns or typical morphology of intrapulmonary lymph nodes 1
• Don't forget that in younger patients (<35 years), infectious causes of nodules are more likely than cancer 1

In conclusion, solid lung nodules detected on CT scans commonly represent healed granulomata from previous infections, particularly in regions with endemic fungal infections. Their benign nature can often be inferred from specific imaging characteristics, stability over time, and patient risk factors.