Pulmonary metastasis results from the spread of an extrapulmonary malignant tumor through vascular or lymphatic channels or by direct extension. Almost any cancer can metastasize to the lung, including primary lung carcinomas. Metastases usually occur via the pulmonary artery and typically present as multiple nodules or masses on chest radiography. The radiographic differential diagnosis of multiple pulmonary nodules also includes pulmonary arteriovenous malformation, infections (including abscesses, septic emboli, and atypical infections), sarcoidosis, rheumatoid nodules, and granulomatosis with polyangiitis. Metastases to the lungs are found in 20–55% of patients with various metastatic malignancies. Most are intraparenchymal. Carcinomas of the kidney, breast, rectum, colon, and cervix and malignant melanoma are the most likely primary tumors. Head and neck cancers with extensive or lower cervical nodal involvement have a 30% risk for distant metastasis; half of these metastases present in the lungs. Pulmonary metastases are common in patients with osteosarcomas and develop in approximately 20% of patients with soft tissue sarcomas.
Lymphangitic carcinomatosis denotes diffuse involvement of the pulmonary lymphatic network by primary or metastatic lung cancer, probably a result of extension of tumor from lung capillaries to the lymphatics. Tumor embolization from extrapulmonary cancer (renal cell carcinoma, hepatocellular carcinoma, choriocarcinoma) is an uncommon route for tumor spread to the lungs. Metastatic cancer may also present as a malignant pleural effusion.
Symptoms are uncommon but include cough, hemoptysis and, in advanced cases, dyspnea and hypoxemia. Symptoms are more often referable to the site of the primary tumor.
The diagnosis of metastatic cancer involving the lungs is usually established by identifying a primary tumor. Appropriate studies should be ordered if there is a suspicion of any primary cancer, such as breast, thyroid, testis, colorectal, or prostate, for which specific treatment is available. For example, an elevated prostate-specific antigen (PSA) may indicate prostate cancer, and beta-human chorionic gonadotropin (beta-hCG) and alpha-fetoprotein point to germ cell tumors. Based on the clinical setting, imaging studies should be ordered. If the history, physical examination, and initial studies fail to reveal the site of the primary tumor, attention is better focused on the lung, where tissue samples obtained by bronchoscopy, percutaneous needle biopsy, video-assisted thoracoscopic surgery (VATS), or thoracotomy may establish the histologic diagnosis and suggest the most likely primary cancer. Occasionally, cytologic studies of pleural fluid or pleural biopsy reveals the diagnosis.
To determine a primary diagnosis, immunohistochemical staining should be done on the biopsy specimen. For example, prostate-specific antigen (PSA) and thyroglobulin staining are highly specific for prostate and thyroid cancer, respectively. Thyroid transcription factor-1 (TTF-1) and napsin-A are relatively specific for primary lung adenocarcinoma, while the former can be positive in cases of SCLC and thyroid carcinoma and the latter can be positive in papillary and clear cell renal cell carcinomas. An adenocarcinoma that demonstrates negative TTF-1 and napsin-A staining ...