Fibrosarcoma Originating from the Pleura in Cotton-Top Tamarin [Oedipomidas (Saguinus) oedipus, Linnaeus 1758]

This report describes postmortem findings after the sudden death of a captive-bred middle-aged male cotton-top tamarin (<i>Saguinus oedipus</i>). A firm structure was attached to the pleura in the chest cavity and was assessed as a primary tumor without obvious metastasis. Histologically, the mass showed heterogenicity with immunohistochemistry (IHC) reactivity to vimentin, suggesting a diagnosis of fibrosarcoma of mesenchymal origin.