Thymoma originating from the cervical component of the thymus in a degu

A 5-y-old, male degu ( Octodon degus) was presented with a subcutaneous mass in the ventral aspect of the cervical area. The mass was removed surgically. Histologically, the mass was a densely cellular, expansile neoplasm, with compression of thymic tissue to the periphery. The neoplasm consisted of solid sheets of polygonal cells, mixed with fewer small lymphocytes. Rare Hassall bodies were scattered throughout the mass. Polygonal cells were positive for anti-keratin/cytokeratin AE1/AE3 antibody, and small lymphocytes were positive for anti-CD3 antibody. The histopathologic and immunohistochemical findings were consistent with a thymoma. In addition, an autopsy revealed myxosarcoma of the right thoracic wall with metastasis to the lung. To our knowledge, thymoma originating from the cervical component of the thymus has not been documented previously in a rodent species.