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The new World Health Organization classification of lung tumours

¹ Service de Pathologie Cellulaire, Centre Hospitalier Universitaire de Grenoble, Grenoble, France. ² Dept of Pulmonary and Mediastinal Pathology, Armed Forces Institute of Pathology, Washington, USA. ³ Dept of Pathology, Mayo Clinic, Scottsdale, USA. ⁴ Dept of Histopathology, Royal Brompton Hospital, National Heart and Lung Hospital, London, UK. ⁵ Clinical Laboratory Division, National Cancer Center Hospital, Tokyo, Japan

CORRESPONDENCE: E. Brambilla, Service de Pathologie Cellulaire, Centre Hospitalier Universitaire de Grenoble, B.P. 217-38043 Grenoble Cedex, France. Fax: 33 476765949

Keywords: histological classification, lung tumours, World Health Organization

Received: July 21, 2001
Accepted July 22, 2001

Abstract

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 1) include the addition of two pre-invasive lesions to squamous dysplasia and carcinoma in situ; atypical adenomatous hyperplasia and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia. Another change is the subclassification of adenocarcinoma: the definition of bronchioalveolar carcinoma has been restricted to noninvasive 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 LCNEC and the basaloid carcinoma, both with a dismal prognosis. Finally, a new class was defined called carcinoma with pleomorphic, sarcomatoid, or sarcomatous elements, which brings together 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 could be classified by light microscopic criteria.