Spontaneous malignant myoid thymoma in an aged female Fischer 344 rat

Abstract

A whitish mass approximately 30 mm in diameter was noted in the anterior mediastinum of a 67-week-old female Fischer 344 rat. Histopathologically, two types of tumor cells were identified on the basis of morphologic features: epithelial tumor cells with a tubular or cord-like growth pattern and rhabdomyosarcomatous tumor cells characterized by the presence of cross-striations. Immunohistochemically, the epithelial tumor cells reacted positively for cytokeratin AE1/AE3, and some reacted positively for p63, which is expressed in normal thymic epithelial cells. The rhabdomyosarcomatous tumor cells stained positively for desmin, sarcomeric actin, and S-100 protein, which coincides with the stainability of normal thymic myoid cells. Since the tumor was also found to have malignant features such as high proliferative activity, cytologic atypia, and necrotic behavior, it was diagnosed as a malignant myoid thymoma. We believe that this is the first case report of such a tumor in a rodent.

Keywords: Fischer rat; epithelial; malignant myoid thymoma; rhabdomyosarcomatous.

Fig. 1.

Gross appearance of a mass detected in an aged female Fischer 344 rat. The mass is located in the mediastinum, corresponding to the thymus region, and measures 30 mm in diameter.

Fig. 2.

Hematoxylin and eosin-stained photomicrographs of the tumor. (a) Epithelial tumor cells showing a tubular or cord-like growth pattern. The cell density is high in the area of the tubular growth pattern (left) and low in the area of the cord-like growth pattern (right). Both components are adjacent to each other with a relatively clear border (×80). (b) Rhabdomyosarcomatous tumor cells (eosinophilic area) and undifferentiated tumor cells (basophilic area) are intermingled (×80). (c, d) Higher magnification of tubular (c) and cord-like (d) growth patterns. Both types of tumor cells are cuboidal and have oval nuclei and basophilic cytoplasm. The stroma of the cells with the cord-like growth pattern is edematous, and scattered lymphocytes and macrophages are present (×320). (e) Large pleomorphic rhabdomyosarcomatous cells have a few round to bizarre nuclei with a few nucleoli and eosinophilic cytoplasm (×320). (f) Spindle rhabdomyosarcomatous cells have oval nuclei and eosinophilic cytoplasm. Inset: Cross-striations visible on PTAH staining (×320). (g) Higher magnification of undifferentiated tumor cells. The tumor cells are polygonal with oval nuclei and show a solid proliferation pattern. Mitotic figures are frequently observed (×320).

Fig. 3.

Immunohistochemical photomicrographs of the tumor (×320). (a, b) Cytokeratin AE1/AE3 immunostaining. Epithelial tumor cells with tubular (a) and cord-like (b) growth patterns react positively for cytokeratin AE1/AE3, but the reactivity is stronger in the cord-like growth pattern. (c) p63 immunostaining. Some epithelial tumor cells with the cord-like growth pattern react positively for p63. (d) Sarcomeric actin immunostaining. Spindle rhabdomyosarcomatous tumor cells stain positively for sarcomeric actin, and cross-striations in the cytoplasm are clearly seen. Epithelial tumor cells (arrowheads) are also observed. Inset: Enlarged image showing cytoplasmic cross-striations. (e) Desmin immunostaining. Parts of the cytoplasm in undifferentiated tumor cells stain positively for desmin. A large pleomorphic rhabdomyosarcomatous tumor cell (arrowhead) can be seen.