Appraisal of the World Health Organization classification of lung tumours.

The 1967 World Health Organization (WHO) classification of the histologic types of lung tumours has been examined in relation to 303 tumours of patients admitted to a national trial of the treatment of lung cancer. In 63 instances (20%) there was disagreement between the diagnoses of the local pathologist and the reference pathologist. The slides for these 63 tumours were reviewed by two other pathologists, and the slides for 60 tumours for which there was agreement between the local and reference pathologists were reviewed by one of the other pathologists. The main disagreement was in the diagnosis of epidermoid (squamous) tumours. It is apparent that many pathologists do not adhere to the strict criterion of the presence of keratinization or intercellular prickles or both for the diagnosis of epidermoid carcinoma. In addition, there was substantial variation in the use of subtypes within the WHO classification. Use of the revised classification proposed by the WHO would have removed a small amount of the variation from these findings but would not have affected the main discrepancy. Stricter attention to the definition of types is required for a uniform approach to the histologic classification of lung tumours.