[Pathology of early lung cancer].

Lung cancer is divided into hilar and peripheral types by its site of origin, because of the difference in the structures between bronchi and bronchiolo-alveoli. Accordingly, the definition of early lung cancer differs depending upon the site of tumor origin. At present, however, there is no agreement as to the definition of early lung cancer of both types. Most early cancer of the hilar region are squamous cell type. Squamous cell carcinoma in situ should be differentiated from dysplasia with marked atypia. On the other hand, many early cancer of the peripheral type are adenocarcinoma. Atypical adenomatous hyperplasia or adenoma may necessitate differentiation from very well differentiated adenocarcinoma. Problems on the definition and differential diagnosis are discussed.