Nonsystemic locally invasive thymic T-cell lymphoma in a fifteen-week-old male CD-1 mouse.

A male CD-1 mouse from a segment I reproduction toxicity study died at the age of fifteen weeks. At necropsy, the thymus was abnormally enlarged and virtually obliterated the thoracic cavity. Histopathological examination of thymus revealed abundant cell population consisting of mature lymphocytes and few lymphoblasts arranged in cords and sheets. Neoplastic cells completely obliterated the thymic architecture, and loss of corticomedullary junction was evident. Microscopically, neoplastic cells were round with moderate cytoplasm. Nuclei were spherical and hyperchromatic with centrally placed nucleoli. Mitotic figures and tingible body macrophages imparting "starry sky appearance" were seen. Multifocal necrotic areas were also observed. Proliferating neoplastic cells invaded the thymic capsule, surrounding adipose tissue and neighboring organs such as lungs, heart and fat pad of aorta, esophagus, trachea, and thoracic vertebrae. No evidence of systemic metastasis was observed in other organs. Immunostained neoplastic cells were positive for CD3 antigen. Gross, histopathology, and immunohistochemistry results were suggestive of thymic T-cell lymphoma.