Auricular rhabdomyosarcoma in a rat.

An ear (auricular) neoplasm from a 1-year-old male rat was removed surgically and examined histologically. Macroscopically, the neoplasm was firm, white and measured (0.5 x 0.5 cm). Microscopically, the neoplasm was expansile, non-encapsulated, and composed of large, pleomorphic, polygonal to spindle-shaped cells containing multiple nuclei. Using immunohistochemical and chemical stains, the neoplastic cells were positive for vimentin, myoglobin, phosphotungstic acid haematoxylin and desmin, but had no immunoreactivity for cytokeratin or alpha-smooth muscle actin. On the basis of histopathological, immunohistochemical and histochemical stains, a diagnosis of auricular rhabdomyosarcoma was made. Although reported infrequently in human, this is, to the author's knowledge, the first report that describes the detailed gross, histopathological, histochemical and immunohistochemical findings of auricular rhabdomyosarcoma in a rat.