The World Health Organization's Histological Classification of Lung Tumors: a comparison of the first and second editions.

Revision of the WHO Histological Classification of Lung Tumors of 1967 has resulted in the 1981 publication of the second edition. The main features of the revisions have been summarized. Squamous cell carcinoma (epidermoid carcinoma) has the same definition as in the original version, ie, the identification of keratin and/or intercellular bridges by light microscopy. Three degrees of histological differentiation have been described. Dysplasia and carcinoma in situ have been discussed. Small cell carcinoma has been divided into oat cell carcinoma, an intermediate cell type and a category for oat cell carcinomas combined with other major types. Adenocarcinoma includes the acinar, papillary and bronchiolo-alveolar forms, and the solid carcinomas with mucus formation (previously part of the large cell carcinoma group). A number of less common tumors and tumor-like lesions have been defined. None of the changes in the revision are considered to be a major departure from the original classification. This is intended in order to preserve comparability between data collected with each edition. The need for utilizing standardized nomenclature, diagnostic criteria, and tabulation formats is stressed.