Spontaneous pericardial mesothelioma in a rhesus monkey

Spontaneous tumors in nonhuman primates are of great importance. A spontaneous pericardial mesothelioma was observed in an 18-year-old female rhesus monkey. Grossly, the visceral pericardium was multifocally irregular and thickened with tan discoloration and was soft in consistency. Histologically, the pericardium contained highly infolded branching fronds lined by a single layer of cuboidal cells. Tumor invaded into approximately half of the thickness of the atrial and ventricular muscles. Tumor penetration was not observed into the atrial or ventricular cavity. Within the myocardium, neoplastic cells formed glandular structures which were lined by cuboidal to columnar cells. Neoplastic cells were weakly positive with PAS and strongly positive for colloid iron and alcian blue. Immunohistochemically, neoplastic cells were positive for both vimentin and cytokeratin and negative with CEA and Leu-Mi, indicating mesothelial origin. To the best of the authors' knowledge, this is the first report of a spontaneous pericardial mesothelioma in a rhesus monkey.