Epidermoid cysts in the central nervous system of rats and mice: an incidental finding in toxicity/oncogenicity studies.

muscle. Three major histologic types are described: pleomorphic, embryonal, and alveolar.' The pleomorphic type arises from myotome-derived skeletal muscle (adult variety), the embryonal type originates from undifferentiated mcsoderm (juvenile variety), and the alveolar type is related to the embryonal type. Alveolar rhabdomyosarcoma has been reported in dogs, either as locally infiltrating or advanced neoplasm. Although unusual sites of origin (orbital and sinonasal regions, abdominal wall) have been occasionally indicated. no primary omental cases have been reported. Microscopically. the typical alveolar feature generally leads to a correct diagnosis. even in the absence of important findings such as cross-striations, which can be difficult to demonstrate in poorly differentiated cases. Immunohistochemistry is a useful tool to confirm the histologic diagn0sis.j Positive immunostaining for myoglobin, a specific protein for striated muscle differentiation, is definitely diagnostic. In the present canine case, all histologic and immunohistochemical features suggested the diagnosis of alveolar rhabdomyosarcoma. References