What causes pulmonary nodules? — Department of Respiratory Medicine
Lung nodules are small, round or oval-shaped abnormalities detected on chest imaging—typically measuring less than 3 cm in diameter. They represent localized areas of increased density within the lung parenchyma and are often discovered incidentally during chest X-rays or CT scans performed for unrelated reasons. The underlying causes are diverse and broadly categorized as benign or malignant.
Benign causes are far more common—especially for nodules smaller than 8 mm—and include infectious etiologies such as granulomas from prior tuberculosis or fungal infections (e.g., histoplasmosis, coccidioidomycosis), organizing pneumonia, or residual inflammatory changes following bacterial or viral pneumonias. Non-infectious inflammatory conditions like sarcoidosis, rheumatoid arthritis, or granulomatosis with polyangiitis may also manifest as pulmonary nodules. Additionally, benign neoplasms—including hamartomas (the most common benign lung tumor), carcinoid tumors (typically low-grade and slow-growing), and sclerosing hemangiomas—can present as solitary or multiple nodules.
Malignant causes must always be considered, particularly in individuals with risk factors such as tobacco use, advanced age (>65 years), personal or family history of cancer, or exposure to occupational carcinogens (e.g., asbestos, radon, silica). Primary lung cancers—most commonly non-small cell lung carcinoma (adenocarcinoma, squamous cell carcinoma) or small cell lung carcinoma—may appear as solitary pulmonary nodules. Metastatic disease from extrapulmonary malignancies (e.g., colorectal, breast, renal, or melanoma) is another important consideration, especially when multiple nodules are present.
Other less common but clinically relevant etiologies include vascular lesions (e.g., arteriovenous malformations), amyloid deposits, and focal interstitial fibrosis. Evaluation requires integration of nodule characteristics (size, shape, margin, density—solid vs. subsolid vs. ground-glass), growth kinetics on serial imaging, patient-specific risk factors, and occasionally PET-CT or tissue sampling via biopsy or surgical resection. Management follows evidence-based guidelines such as those from the Fleischner Society or the American College of Chest Physicians, emphasizing risk-stratified surveillance or timely diagnostic intervention.