Lung cancer is the most common cancer worldwide, with about 1.8 million new cases and 1.6 million deaths in 2012 . (See "Overview of the risk factors, pathology, and clinical manifestations of lung cancer".)
The pathologic features of the major lung malignancies will be reviewed here. Clinical features, diagnosis, and management of patients with the different lung malignancies are discussed in the appropriate topic reviews. (See "Overview of the risk factors, pathology, and clinical manifestations of lung cancer" and "Overview of the initial evaluation, treatment and prognosis of lung cancer".)
Classification of lung carcinomas by histopathologic subtype provides important information about prognosis and is necessary for optimal treatment. Traditionally, the classification of lung carcinoma has been based solely on evaluation of routinely stained biopsies or cytologic preparations. Increasingly, however, ancillary tests such as immunohistochemistry are being used to aid pathologists in diagnosis of subtypes. The classification scheme for lung carcinoma is therefore in a state of flux with the 2004 World Health Organization (WHO) providing the framework for classification, but the tools used to arrive at the diagnosis now supersede what that classification scheme endorsed.
The 2004 World Health Organization (WHO) schema of lung tumors has been the foundation for lung cancer classification (table 1) . This incorporated a number of developments, including recognition of lung carcinoma heterogeneity, the introduction of diagnostic immunohistochemical staining (IHC) techniques for the routine diagnosis of some neuroendocrine tumors, and the recognition of newly described entities such as fetal adenocarcinoma, cystic mucinous tumors, and large cell neuroendocrine carcinoma.
In 2011, a multidisciplinary expert panel representing the International Association for the Study of Lung Cancer (IASLC), the American Thoracic Society (ATS), and the European Respiratory Society (ERS) proposed a major revision of the classification system . These changes primarily affect the classification of adenocarcinoma and its distinction from squamous cell carcinoma.