Canine cardiac rhabdomyoma

A well-circumscribed, expansile, and nonencapsulated cardiac rhabdomyoma composed of tightly arranged, large, variably sized, ovoid to irregular, swollen myocytes with deeply eosinophilic cytoplasm and varying degrees of cytoplasmic vacuolation was detected in an eight- to nine-month-old female beagle dog in a routine toxicology study. By histochemistry, the neoplasm was periodic acid-Schiff positive. By immunohistochemistry (IHC), neoplastic cells were positive for desmin and myoglobin and negative for vimentin and smooth muscle actin. Spontaneous lesions in the heart of young beagle dogs are rare in drug safety studies. On the basis of histopathology, histochemistry, and IHC findings, a diagnosis of cardiac rhabdomyoma was made. Cardiac rhabdomyoma is one of the most frequently occurring primary tumors of the heart and, by far, the most common neoplasm in human infants and children. To our knowledge, this is the first report of the canine cardiac rhabdomyoma.