Basal cell (basaloid) carcinoma of the lung: a new morphologic and phenotypic entity with separate prognostic significance.

On review of 115 poorly or undifferentiated lung cancers from 671 lung tumors resected over a 7-year period, we have found 38 cases of basaloid carcinoma. The cardinal histopathologic features distinguishing this tumor from other non-small cell lung cancers are a lobular growth pattern of small cells with moderately hyperchromatic nuclei, with no prominent nucleoli, and with scant cytoplasm, a high mitotic rate, and peripheral palisading. Basaloid carcinoma was present in a pure form in 19 cases and the other 19 tumors were of a mixed, but prominent, basaloid type associated with squamous cell carcinoma, large cell carcinoma, or adenocarcinoma. The immunophenotype of basaloid cancers was close to that of basal bronchial epithelial cells, with a low level of expression of low molecular weight cytokeratins. Staining for neuroendocrine markers was infrequent and inconsistent. Ultrastructural study showed an absence of neurosecretory granules and the presence of some squamous and/or glandular differentiation. This morphologic and immunologic phenotype suggests that basaloid carcinoma is derived from a pluripotent reserve cell or a basal bronchial epithelial stem cell. This unique histologic form of lung tumor has a poor prognosis, with a median survival rate of 22 months for stage I and II disease. This justifies classification of basaloid carcinoma as a distinct form of lung cancer, separate from small cell lung carcinoma.