Immunostaining in lung cancer for the clinician. Commonly used markers for differentiating primary and metastatic pulmonary tumors.

Immunohistochemical stains have become invaluable for the diagnosis of pulmonary malignancies (both primary and metastatic), particularly given the small size of transbronchial and endobronchial biopsies and the increasing need to conserve tissue for molecular studies. There are many panels of immunostains currently available to help differentiate between common types of pulmonary malignancies. It is the purpose of this review to summarize some of the most commonly used immunostains for the distinction of primary pulmonary malignancies from one another in areas with histological overlap and for distinguishing primary pulmonary malignancies from other cancers with which they may be confused. These include differentiating between poorly differentiated adenocarcinoma and poorly differentiated squamous cell carcinoma, small cell/large cell neuroendocrine carcinoma and basaloid carcinoma, and primary and metastatic adenocarcinoma involving the lung. In addition, we address the distinction between mesothelioma and pulmonary adenocarcinoma. Pitfalls in the use of these markers are also addressed. Although not aiming to be comprehensive, this review aims to guide and influence common practice by furthering the clinician's knowledge on using immunohistochemical stains for characterization of pulmonary neoplasms. This summary of frequently used immunohistochemical stains can provide usefulness by allowing accurate characterization of pulmonary tumors, thereby allowing for conservation of tissue for additional molecular testing.