Mixed form of pericardial mesothelioma with osseous differentiation in a dog.

An 8-year-old female Yorkshire terrier was referred for evaluation and treatment of recurrent pericardial effusion. Echocardiographic examination revealed a markedly and irregularly thickened pericardial sac with frequent hyperechoic areas with acoustic shadows. Pericardiocentesis produced only a small amount of thick serosanguineous fluid. The dog underwent subtotal pericardiectomy, but died during surgery. At necropsy examination, the heart was encased by voluminous, grey-white to red-tan, soft to firm proliferative tissue arising from the pericardial sac. The pericardial cavity was obliterated. Microscopically, the tissue was predominantly sarcomatoid with osseous differentiation and epithelioid elements were admixed with bundles of spindle cells. Immunohistochemically, the constituent cells, especially those that were epithelioid, co-expressed cytokeratin and vimentin. A diagnosis of mixed form pericardial mesothelioma with osseous differentiation was made. This appears to be the first report of such a tumour in a dog.