Pathology of lung cancer

Microscopic examination of stained smears and tissue sections remains the s tandard method for definitive diagnosis and classification of lung cancer. However, the morphology of lung cancer is complex, and consensus classifica tions such as those prepared by a panel World Health Organization (WHO) are required for the sake of consistency and clinical relevance. In the most r ecent (1999) WHO classification, the diagnostic categories of greatest clin ical importance, small cell lung carcinoma and non-small cell lung carcinom a, remain fundamentally unchanged. However, application of immunohistochemi stry and electron microscopy has revealed expression of neuroendocrine mark ers in a wide variety of tumors. Expression of these markers is not taken i nto account in current treatment protocols, and additional correlative stud ies will be required to determine the clinical relevance of neuroendocrine differentiation in lung carcinoma. In addition to histological classificati on, microscopic analysis can provide in situ evidence of response to chemot herapy, as well as information on precursor lesions and multistep carcinoge nesis in the airways. Finally, it is likely that morphological assessment o f lung carcinoma and preneoplastic lesions will continue to be refined as n ew diagnostic modalities such as spiral computed tomography and fluorescenc e bronchoscopy provide previously inaccessible specimens for morphological cal and correlative molecular studies.