Classification of Lung Tumors

Abstract

The first classification of lung tumors was published in 1967 by the International Association for the Study of Lung Cancer under the auspices of the World Health Organization. It was updated for the first time in 1981. The third edition of the same classification was published in 1997. The last published WHO classification was in 2004. The WHO adopts a policy of making any classification based on methods and criteria that are easy to apply and reproduce in any setting anywhere in the world. For histopathologic diagnosis, hematoxylin and eosin (H&E)-stained histologic sections are the standard type of morphology that is followed for classification. The role of ancillary studies such as immunohistochemistry or molecular markers should be to confirm not to make the diagnosis. Differentiation along the epithelial vs. mesenchymal types of tissue would serve as a basis to differentiate most carcinomas from sarcomas. Within the epithelial category, carcinomas would be distinguished according to their further differentiation along glandular or squamous lineage. Undifferentiated carcinoma with neuroendocrine differentiation is reserved for small carcinoma with specific clinical and molecular implications. Combined or mixed differentiation could also occur. The majority of tumors in the lung are carcinomas (90–95 %), with the remainder 5 % representing bronchial carcinoid and 2–5 % representing mesenchymal or other miscellaneous tumors.