Giant cell carcinoma of the lung--immunohistochemical and ultrastructural evidence of dedifferentiation.

Ten examples of giant cell carcinoma of the lung were examined by immunohistochemistry for expression of keratin and vimentin intermediate filaments and for epithelial membrane antigen (EMA). Six cases were also examined electron microscopically. Keratin expression and, to a lesser extent, EMA immunoreactivity were reduced in comparison with better differentiated forms of lung carcinoma. Vimentin expression was increased, often taking the form of strong paranuclear staining. This may correspond to dense paranuclear aggregates of intermediate filaments seen ultrastructurally. Desmosomes were absent or sparse in most tumours. We propose that giant cell carcinoma arises by a process of dedifferentiation. The resulting loss of epithelial features gives rise to neoplastic cells which have features in common with some forms of sarcoma.