Cytologic Features of Cancers Metastatic to the Lung and Diagnostic Usefulness of Immunohistochemistry: Distinction Between Primary and Secondary Lung Tumors

The lungs are one of the most common visceral sites for metastatic disease. The identification of a metastasis from a second primary lung tumor is clinically important for patients with pulmonary metastases of an extrathoracic origin. Although the cytologic features of metastatic tumors involving the lung have been extensively described, making the cytologic diagnosis is usually not easy in the absence of clinical information. However, the immunohistochemical staining for many tumor markers and the different expressions of cytokeratin 7 and 20 are very useful in the diagnosis. This review presents the cytomorphological spectrum of metastatic tumors along with the immunohistochemical findings.