Comparative Morphologic and Immunohistochemical Investigation of Spontaneously Occurring Thymomas in a Colony of European Hamsters

This study documents the characteristics of a large series of spontaneously occurring thymomas in a laboratory colony of European hamsters (Cricetus cricetus). Thymomas are rare organotypic neoplasms originating from the thymic epithelial compartment. Because the hamster thymomas largely resembled their human counterparts, the recent World Health Organization (WHO) classification of human thymic epithelial tumors was used. Forty hamsters of both sexes aged 3-29 months were examined macroscopically and histo- logically. In 22 (55%) of the 40 animals, necropsy revealed enormous whitish masses in the anterior medias- tinum, with a diameter ranging from 0.5 to 4.5 cm and a lobulated structure. The anatomy of the thymus region was normal in the remaining 18 hamsters. Histologically, the tumors presented as thymuslike organoid structures with areas of medullary and cortical differentiation and a predominance of lymphoid cells. A network of epithelial cells in the cortical areas, demonstrated immunohistochemically with a cross-reactive antibody against pancytokeratin, supported the diagnosis of thymoma. Cortical lymphocytes showed positive staining with cross- reacting antibodies against CD3 and terminal deoxynucleotidyl transferase, characteristic of immature T cells. On the basis of these findings, the tumors were classified as B1 thymomas, in some cases with AB or B2 components, according to the new WHO classification for human thymic epithelial tumors. Several possible differential diagnoses must be kept in mind when tumorous masses are found in the an- terior mediastinum. The most frequent neoplasms are lymphomas, followed by thymomas, tumors of ectopic thyroid tissue, and neuroendocrine or mesenchymal tu- mors. Thymomas are rare malignancies both in human and veterinary medicine. They have been described in a wide range of domestic and laboratory animals and occur more frequently in older animals. In humans, thymomas usually present in the fourth to fifth decade but can occur at all ages without sex predisposition. 28