There are a multitude of benign and malignant processes that may present as an SPN on a chest radiograph. Table 1 lists the more common diagnoses, though there are many processes that uncommonly present in this manner. More recently, studies utilizing CT have reduced the number of benign nodules resected to 20-40 percent. The most common benign causes of SPNs are granulomas from histoplasmosis, coccidioidomycosis, and mycobacteria. Hamartomas are the most common benign neoplasms and constitute approximately 10 percent of benign nodules resected.
Bronchogenic carcinoma is, by far, the most common malignant lesion in surgical series of SPNs.
Metastatic lesions from nonlung primary tumors constitute about 10-30 percent of resected malignant nodules. The most frequent sources of metastasis are squamous carcinomas of the head and neck, and adenocarcinomas of the breast, kidney and colon. Patients with SPN rarely have symptoms attributable to the nodule, and so the detection of the SPN is usually serendipitous. The plain chest roentgenogram usually defines the presence and appearance of the SPN unless discovered on CT or other radiographic imaging performed for another purpose. The importance of reviewing previous chest radiographs cannot be overemphasized.