With the frequent use of chest radiography, solitary pulmonary nodules ("coin lesions") are frequently found in patients without pulmonary symptoms. These lesions pose a diagnostic problem for clinicians since they may represent something as benign as a nipple shadow or as malignant as lung cancer (Figure). The overall incidence of cancer in coin lesions is as low as 10%. Other diagnostic possibilities include (1) infections due to mycobacteria (tuberculosis), fungi (histoplasmosis, coccidioidomycosis), and helminths (echinococcosis); (2) inflammatory nodules from rheumatoid arthritis, focal pneumonitis, and Wegener's granulomatosis; (3) congenital anomalies, such as bronchogenic cysts and arteriovenous malformations; (4) benign neoplasms, eg, hamartomas, hemangiomas, papillary tumors, fibrous tumors of the pleura; (5) malignant neoplasms of the lung; and (6) miscellaneous processes, eg, hematomas, pulmonary infarcts, pleural plaques, loculated effusions, chest wall masses, and mucoid impaction. Although certain radiographic findings may suggest malignancy or benignity, solid pathologic proof that the nodule does not represent a malignancy rests with the clinician. In general, malignant neoplasms are larger and grow rapidly, appear spiculated, often with surface umbilication or notching and eccentric excavation. In addition, cancers often occur in smokers (or former smokers) over the age of 40 with negative skin tests for tuberculosis, histoplasmosis, or coccidioidomycosis (although positive tests do not exclude cancer), and in nodules that lack calcium (CT Hounsfield units < 175). In contrast, benign lesions are small (< 1 cm), stable (> 2 years), and calcified ("target" or "popcorn" distribution; CT Hounsfield units > 175) and are associated with positive skin tests in 70–90% of patients. Evaluation of these patients usually includes chest CT scan, but sputum cytology, cultures, bronchoscopy, and mediastinoscopy are sometimes helpful. FDG-PET scanning plays an important role in the evaluation of tumors that are suspicious for malignancy. PET scan can often differentiate among lesions suspicious for malignancy.
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