Spontaneous Benign Thymoma in a 10-Week-Old Female ICR (CD-1) Mouse.

Thymomas are tumors originating from thymic epithelial cells and are uncommon in animals, including pets and domestic and laboratory animals. A 10-wk-old female ICR (CD-1) mouse in the control group of a toxicity study presented a thymic mass consisting of epithelial and lymphocytic components. Histopathological evaluation showed epithelioid neoplastic cells intermixed with small lymphocytes and areas of hemorrhage. Immunohistochemistry revealed positivity for proliferating cell nuclear antigen and pan-cytokeratin (AE1/AE3), and negativity for S100 and vimentin. Based on these features, the mass was diagnosed as a benign thymoma. To the best of our knowledge, this is the first report of a thymoma in a laboratory mouse at such a young age.