[Classification of broncho-pulmonary cancers (WHO 1999)]

Tumour classification systems provide the foundation for tumour diagnosis and patient therapy and a critical basis for epidemiological and clinical studies. This updated classification was developed with the aim to adhere to the principles of reproducibility, clinical significance, and simplicity in order to minimize the number of unclassifiable lesions. Major changes in the revised classification as compared to the previous one (WHO 1981) include the addition of two pre-invasive lesions to squamous dysplasia and carcinoma in situ: atypical adenomatous hyperplasia (AAH) and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia. Another change is the subclassification of adenocarcinoma: the definition of bronchioloalveolar carcinoma has been restricted to non-invasive tumours. There has been substantial evolution of concepts in neuroendocrine lung tumour classification. Large cell neuroendocrine carcinoma (LCNEC) is now recognized as a histologically high-grade non-small cell carcinoma showing histopathological features of neuroendocrine differentiation as well as immunohistochemical neuroendocrine markers. The large cell carcinoma class has been enriched with several variants, including the large cell neuroendocrine carcinoma and the basaloid carcinoma, both of which have a poor prognosis. Finally, a new class has been defined called carcinoma with pleomorphic, sarcomatoid, or sarcomatous elements, which gathers a number of proliferations characterized by a spectrum of epithelial to mesenchymal differentiation. Immunohistochemistry and electron microscopy are invaluable techniques for diagnosis and subclassification, but our intention was to render the classification simple and practical to every surgical laboratory so that most lung tumours can be classified by light microscopic criteria.