Spontaneous pericardial mesothelioma in a rhesus monkey

Spontaneous tumors in nonhuman primates are of great importance. A spontane ous pericardial mesothelioma was observed in an 18-year-old female rhesus m onkey. Grossly, the visceral pericardium was multifocally irregular and thi ckened with tan discoloration and was soft in consistency. Histologically, the pericardium contained highly infolded branching fronds lined by a singl e layer of cuboidal cells. Tumor invaded into approximately half of the thi ckness of the atrial and ventricular muscles. Tumor penetration was not obs erved into the atrial or ventricular cavity. Within the myocardium, neoplas tic cells formed glandular structures which were lined by cuboidal to colum nar cells. Neoplastic cells were weakly positive with PAS and strongly posi tive for colloid iron and alcian blue. Immunohistochemically, neoplastic ce lls were positive for both vimentin and cytokeratin and negative with CEA a nd Leu-Mi, indicating mesothelial origin. To the best of the authors' knowl edge, this is the first report of a spontaneous pericardial mesothelioma in a rhesus monkey.