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Emergency and Acute Medicine – Lung Nodules
Overview and Definitions
Pulmonary nodules are rounded opacities surrounded by lung parenchyma and are typically less than 3–4 cm in diameter; larger lesions are classified as lung masses. Pathologically, a pulmonary nodule is a small, approximately spherical, circumscribed focus of abnormal tissue, while radiologically it is defined as a round opacity, at least moderately well marginated, and no more than 3 cm in maximum diameter. Linear or planar opacities are not considered nodules. Chest imaging may reveal a solitary pulmonary nodule, often referred to as a coin lesion, or multiple pulmonary nodules.
Clinical Approach
The primary objective is to distinguish benign from malignant nodules. Evaluation begins with a thorough history, as the probability of malignancy increases with age and is rare in patients younger than 35 years. Important malignancy risk factors include prior lung cancer, family history of lung cancer, cigarette smoking, second-hand smoke exposure, occupational carcinogen exposure such as asbestos or radon, pulmonary fibrosis, chronic obstructive pulmonary disease, and alpha-1 antitrypsin deficiency. A higher cumulative smoking burden is associated with increased cancer risk. If malignancy is suspected, it is essential to determine whether the lesion represents a primary lung cancer or metastatic disease. Fever suggests an infectious etiology, and immunocompromised patients are at risk for opportunistic infections presenting as solitary or multiple nodules. Physical examination may reveal signs of malignancy including cachexia, anemia, jaundice, clubbing, bone pain, abdominal masses, or superior vena cava syndrome. Radiologic features such as size, distribution, shape, calcification pattern, cavitation, lymphadenopathy, pleural effusion, and clustering provide diagnostic clues. Low-risk nodules may be followed with serial CT imaging, whereas larger or suspicious nodules warrant biopsy, PET imaging, or video-assisted thoracoscopic surgery. Empiric antimicrobial therapy should be initiated in immunocompromised patients or those at risk for rapid deterioration.
Epidemiology
Pulmonary nodules are common, with up to 51% of smokers aged 50 years or older having nodules detected on CT. Fewer than 1% of nodules smaller than 5 mm are malignant in patients without a history of cancer. The likelihood of malignancy increases with size: approximately 0.2% for nodules under 3 mm, 0.9% for 4–7 mm, 18% for 8–20 mm, and greater than 50% for nodules larger than 20 mm. Male smokers have a tenfold increased risk of lung cancer compared with nonsmokers, rising to 15–35 times higher in heavy smokers. Paragonimus westermani infection can present as a lung nodule in endemic regions of East and Southeast Asia. Tuberculosis commonly presents with hilar lymphadenopathy, and histoplasmosis is the most frequent cause of hospitalization from endemic mycoses in the United States.
Etiology and Pathophysiology
Solitary pulmonary nodules may be caused by primary lung malignancies including adenocarcinoma, squamous cell carcinoma, large cell carcinoma, and small cell carcinoma, as well as metastatic disease from breast, head and neck, thyroid, melanoma, colon, kidney, sarcoma, and germ cell tumors. Other causes include carcinoid tumors, infections such as bacterial abscesses, tuberculosis, atypical mycobacteria, histoplasmosis, blastomycosis, coccidioidomycosis, cryptococcosis, aspergilloma, nocardiosis, Pneumocystis infection, and parasitic infections including echinococcosis, dirofilariasis, ascariasis, and paragonimiasis. Benign neoplasms include hamartoma, lipoma, and fibroma. Non-neoplastic causes include arteriovenous malformations, bronchogenic cysts, granulomatosis with polyangiitis, sarcoidosis, rheumatoid nodules, amyloidoma, rounded atelectasis, intrapulmonary lymph nodes, hematoma, pulmonary infarction, and mucoid impaction. Multiple pulmonary nodules are more often benign when less than 1 cm, but in patients with known malignancy, nodules larger than 0.5 cm are more likely metastatic. Infectious causes of multiple nodules include septic emboli, fungal infections, tuberculosis, atypical mycobacteria, and paragonimiasis, while inflammatory conditions such as sarcoidosis, rheumatoid arthritis, and vasculitis are also common causes.
Diagnostic Evaluation
History should assess for tuberculosis risk factors, immunosuppression, aspiration risk, and exposure to endemic fungi. Lung abscesses may arise from aspiration, tricuspid valve endocarditis, or septic thrombophlebitis such as Lemierre syndrome, often in patients with poor dentition or prolonged recumbency. Symptoms include cough, sputum production, dyspnea, fever, night sweats, weight loss, and anorexia. Laboratory evaluation may include blood cultures, sputum Gram stain and culture, fungal cultures, acid-fast bacilli staining and mycobacterial cultures, cryptococcal antigen testing, galactomannan and beta-D-glucan assays, and serologic testing for endemic fungi. Imaging with high-resolution CT can detect nodules as small as 1–2 mm. Benign calcification patterns include homogeneous, central, concentric, and popcorn calcifications, while malignant features include eccentric or amorphous calcification and spiculated borders. Ground-glass halos suggest invasive fungal infection, particularly aspergillosis. PET imaging identifies metabolically active lesions. Transthoracic echocardiography is indicated when septic emboli are suspected, and CT of the neck should be performed if Lemierre syndrome is considered. Percutaneous biopsy or VATS is indicated when malignancy risk is high.
Management
Empiric antibiotics should be initiated in patients with fever or systemic toxicity after obtaining blood cultures. Suspected tuberculosis requires differentiation between latent and active disease using clinical assessment, imaging, and sputum studies. Mild pulmonary histoplasmosis may not require treatment, but persistent or moderate-to-severe disease is treated with itraconazole or amphotericin B followed by itraconazole. Lung abscesses are typically polymicrobial and treated with beta-lactam/beta-lactamase inhibitors or clindamycin. Voriconazole is first-line therapy for invasive aspergillosis, with surgical debridement considered when feasible. Paragonimiasis is treated with praziquantel or triclabendazole.
Follow-Up and Prognosis
Malignant solitary pulmonary nodules typically double in volume over 20–400 days. Low-risk nodules smaller than 1 cm may be monitored with serial CT imaging, while high-risk nodules should be surgically excised. PET scanning is most useful for nodules larger than 1 cm with intermediate malignancy risk to guide management decisions.
Complications
Untreated lung abscesses may progress to respiratory failure requiring mechanical ventilation. Cavitation may occur in infectious, malignant, and inflammatory nodules, and cavitary lesions are frequently complicated by secondary Aspergillus infection.
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