1995 Volume 8 Issue 4 Pages 441-444
This paper describes a case of primary thymoma found in a 23-week-old young female SD rat. Grossly, the tumor was an encapsulated, grayish, firm mass measuring 20×15×15mm, which was found cranial to the heart in the thoracic cavity. Histologically, the neoplasm consisted of keratinpositive neoplastic epithelial cells and considerably abundant supporting nonneoplastic stroma, which were arranged in storiform and wavy patterns. The neoplastic cells were randomly distributed in the stroma in single or clusters sometimes forming pseudoductular structures. Spindle cells forming the stroma were immunopositive for muscle actin, suggesting features of myofibroblasts. No proliferating lymphocytes were observed in the tumor. Based on these findings, the present case was diagnosed as a benign thymoma, epithelial type with extensive development of collagen fibers. To our knowledge, such a unique thymoma has not been reported in young rats.
