Diagnosis of lung cancer: pathology of invasive and preinvasive neoplasia.

The histopathologic appearance of lung carcinoma remains an important guide to prognosis and treatment. The newly revised World Health Organization classification retains the broadest pathologic categories of the older classification but includes several revisions, including the elimination of the small cell, intermediate cell type category; the addition of large cell neuroendocrine and spindle/giant cell categories; and an extended consideration of preneoplastic lesions. The histopathologic classification of lung cancer is expected to continue to change as clinical practice and biological understanding of these tumors change. The application of immunohistochemical testing to histologic material not only provides new assistance with conventional histologic classification, but also permits new ways to subclassify tumors, the full clinical significance of which is yet to be realized. The significance of expression of neuroendocrine markers, histologic grading of response to chemotherapy, and delineation of morphologic changes preceding the occurrence of invasive carcinoma are all areas where understanding microscopic cellular changes in the airways will be critical for clinical advance.