solitary pulmonary nodule

DEFINITION: Single focal lung opacity ≤3 cm, completely surrounded by lung. Lesions >3 cm are "masses" and are presumed malignant until proven otherwise. Often found incidentally on chest X-ray or CT.

ETIOLOGY: - Benign (~60-70% in older series, depends on prevalence in the region): infectious granulomas (histoplasmosis, TB, coccidioidomycosis), hamartoma (most common benign tumor; "popcorn" calcification, fat density on CT), rheumatoid nodule, granulomatosis with polyangiitis, arteriovenous malformation, bronchogenic cyst, pulmonary infarct, round pneumonia, and intrapulmonary lymph node. - Malignant: primary lung cancer (adenocarcinoma is the most common type presenting as SPN, often peripheral; squamous cell cancer is typically central), carcinoid, and solitary metastasis (colon, breast, sarcoma, melanoma, and renal cell cancer).

FEATURES FAVORING BENIGN: age <40, nonsmoker, size <8 mm, smooth, well-defined margins, and calcification patterns that are central, diffuse/solid, laminated (concentric), or popcorn. Stability (no growth) for ≥2 years on imaging for solid nodules (doubling time <20 days or >400 days suggests benign).

FEATURES FAVORING MALIGNANT: age >40-50, smoking history, prior cancer, family history, asbestos/radon exposure, upper lobe location, size >2-3 cm, spiculated or lobulated margins ("corona radiata" sign), eccentric or stippled calcification, new or growing nodule, and part-solid or ground-glass component (higher risk of adenocarcinoma spectrum: AIS → MIA → invasive adenocarcinoma). Typical malignant volume doubling time is ~20-400 days.

WORKUP: 1. Compare with old imaging (most important and cheapest step). Stability ≥2 years for a solid nodule = benign. 2. Chest CT (thin-section) to characterize size, margins, calcification, fat, and density. 3. Estimate pretest probability of malignancy (e.g., Brock/Mayo models; low <5%, intermediate 5-65%, high >65%). 4. Low risk: serial CT surveillance (Fleischner Society guidelines for incidental nodules, by size and risk). Intermediate risk: PET-CT (malignancy shows increased FDG uptake; false negatives with carcinoid, adenocarcinoma in situ, and small nodules <8 mm; false positives with active infection/granulomas) and/or biopsy. High risk: surgical resection (VATS wedge resection) after staging. 5. Biopsy options: CT-guided transthoracic needle biopsy (peripheral lesions; risk of pneumothorax), bronchoscopy with navigation/EBUS (central lesions), or surgical excision.

MANAGEMENT: Malignant or high suspicion in a surgical candidate = resection (lobectomy preferred for stage I NSCLC; SBRT for nonoperative candidates). Benign = observation.

BOARD PEARLS: - Most common cause of a benign SPN: infectious granuloma. Most common benign neoplasm: hamartoma. - Popcorn calcification = hamartoma. Fat on CT = hamartoma. - Spiculated edge + upper lobe + smoker = think cancer. - Lesion >3 cm = mass, treat as cancer. - Best next step for a newly found nodule: compare to prior imaging, then get a CT. - Lung screening: annual low-dose CT for ages 50-80 with ≥20 pack-year history who currently smoke or quit within the past 15 years (USPSTF).