A histopathological variant of mixed thymoma presenting with prominent calcification and desmoplasia in an aged Japanese Brown cow

Herein an unusual variant of mixed thymoma with extensive fibrosis and calcification is described in a Japanese Brown cow, aged 18 years and 8 months, that was culled from old age. At necropsy, a solitary tumor mass (approximately10 cm in diameter) was found in the cranial mediastinal region. Microscopically, the tumor consisted of two distinct cell types, namely tightly packed sheets of immunohistochemically cytokeratin-positive neoplastic epithelial cells intermingled with CD3-positive T lymphocyte populations of varying density. Immature Hassall's corpuscles, cystoid cavities, and small mineralized bodies (calcispherites) were observed. A particularly characteristic feature that highlighted this tumor was the presence of extensive and multifocal areas of sclerotic and fibroconnective stroma. These areas were frequently associated with calcification undergoing granulomatous reaction. Histopathological and immunohistochemical findings were similar, to some degree, to those described in only a few reports of sclerosing thymoma, a rare variant of thymoma in human beings.