Botryoid-type embryonal rhabdomyosarcoma of liver in a young cat.

An unusual malignant mesenchymal tumor arising in the liver of a 2-year-old cat is described. Histologically, the tumor showed considerable variation in growth pattern, cellularity, and cell types. Phenotypical diversity was confirmed by immunohistochemistry, showing expression of desmin, vimentin, S-100, and neuron-specific enolase in various areas of the tumor. On the basis of histopathology, immunohistochemistry, electron microscopy, and gross morphology, the tumor was classified as botryoid-type embryonal rhabdomyosarcoma. Differential diagnosis included so-called undifferentiated (embryonal) sarcoma of the liver, a rare tumor of the pediatric age group in humans. Problems of tumor heterogeneity and differentiation in mesenchymal tumors are discussed.