Metaplastic squamous cell carcinoma of bronchus simulating giant cell tumor of bone.

A 57-year-old man underwent a right pneumonectomy for a bronchogenic carcinoma following bronchoscopy and bronchial biopsy. The tumor was a polypoid mass arising from the lower lobe bronchus. Microscopically it was characterized by mononuclear cells mixed with randomly distributed multinucleated giant cells similar to those seen in giant-cell tumor of bone. Also found were portions showing typical squamous cell and spindle cell carcinoma. Based on the light and electron microscopic findings, we suggest that the current case represents a metaplastic squamous carcinoma showing mesenchymal cell differentiation. A hypothesis on the histogenesis of pleomorphic carcinomas was presented. The problems of histological diagnosis generated by such a tumor should be emphasized.