Osteoclast-like giant cell tumour of the gallbladder.

We describe a rare carcinoma of the gallbladder containing osteoclast-like giant cells. Well-differentiated adenocarcinoma was found in the mucosa of the fundus, and osteoclast-like giant cells were present mainly in a haemorrhagic mass protruding from the mucosal surface. The metastatic hepatic tumour was composed chiefly, if not exclusively, of osteoclastoma-like cells, but minute carcinomatous elements were also present. There was an apparent transition between the giant cells and tubular structures in both the gallbladder tumour and hepatic tumour. However, ultrastructural study did not reveal any evidence of epithelial differentiation in the giant cells. Immunohistochemical studies suggested that the mononuclear and giant cells were mesenchymal and histiocytic in nature (vimentin and factor XIIIa positive). A few exceptional giant cells transforming from the fine tubular structure were positive for epithelial membrane antigen. In conclusion, the osteoclast-like giant cell tumour component was thought to represent mesenchymal metaplasia in pre-existent adenocarcinoma.