The relevance of ultrastructural examination in the classification of primary lung tumours.

Biopsies from 50 primary lung tumours were classified according to the World Health Organisation's Histological Typing of Lung Tumours. They were also subjected to electron microscopic examination. Comparison of the diagnoses made by these separate methods showed that many poorly differentiated squamous cell carcinomas had been incorrectly classified. Agreement was good in the diagnosis of adenocarcinoma whilst ultrastructural examination of small anaplastic carcinomas disclosed a neuroendocrine tumour with a combination of squamous and glandular elements. Large cell anaplastic carcinoma proved to be a 'waste-basket' containing tumours which displayed ultrastructural characteristics of poorly differentiated squamous cell carcinoma, poorly differentiated adenocarcinoma or neuroendocrine carcinoma. Electron microscopy was also valuable in characterization of other pulmonary tumours whose identity could not be resolved at the light microscopic level. Ultrastructural examination may provide a better understanding of the histogenesis and derivation of lung tumours, as well as their behaviour and therapeutic response.