In order to study the problem of morphological tumor heterogeneity in lung cancer, 200 consecutive patients who had undergone surgery for a malignant lung tumor, were evaluated retrospectively with regard to morphological type. The tumor was classified morphologically in 187 patients, and 163 (87%) had a morphologically homogeneous tumor, based on light microscopic features and using the criteria recommended by the World Health Organization. The remaining 24 patients (13%) had a tumor with morphologic features of more than one cell type.

It is concluded that morphological heterogeneity is a considerable problem in the classification of malignant lung tumors. This heterogeneity has been regarded as evidence of an endodermal origin of all major types of lung cancer. Future prospective studies will have to determine whether it has any therapeutic significance.