Metastatic myxosarcoma in a Quarter Horse gelding

Abstract

A 22-y-old Quarter Horse gelding was presented to the University of Illinois Veterinary Teaching Hospital for evaluation of increased heart rate and mild colic signs. Rectal examination revealed a large left perirenal mass. Abdominal ultrasonography further confirmed this finding. Thoracic ultrasonography indicated multifocal irregularities on the pleural surface suggestive of consolidation and possibly masses in the lungs. The animal was euthanized. Autopsy findings included a large, firm, expansile, gelatinous retroperitoneal mass that surrounded both kidneys, as well as nodules with similar morphology in the lungs, liver, intestinal mesentery, cecum, and caudal mesenteric artery. Histologically, the masses were composed of neoplastic stellate-to-spindloid cells in abundant mucinous stroma. Neoplastic cells exhibited strong immunoreactivity for vimentin and were negative for pancytokeratin (A1/A3), CD3, CD20, melan A, and synaptophysin. Mucinous stroma was strongly positive with alcian blue and weakly positive with periodic acid-Schiff histochemical staining. These findings are consistent with metastatic myxosarcoma. Myxosarcoma is a rare neoplasm in horses, and metastasis to tissues other than sentinel lymph nodes has not been described previously to our knowledge.

Keywords: Equine; horses; metastasis; myxosarcoma; retroperitoneum.

Figure 1.

A. A large multinodular mass within the horse’s retroperitoneal space (arrows). B. Retroperitoneal mass and kidney. The cut surface of the neoplasm is yellow-to-tan-to-red with clear-to-yellow mucinous foci. The neoplasm completely surrounds the left kidney (margins on arrows) and infiltrates the caudal pole (arrowheads). C. Variably sized white-to-tan metastatic neoplastic nodules on the pleural surface (arrows). D. Variably sized white-to-tan-to-red metastatic neoplastic nodules on the surface of the liver (arrows).

Figure 2.

A. Lobules are separated by bands of fibrous connective tissue and are composed of neoplastic cells, eosinophilic, wispy material, and clear, colorless space. H&E. B. Neoplastic cells are stellate-to-spindloid with moderate eosinophilic, fibrillar cytoplasm, and hyperchromatic nuclei. The cells are separated by abundant clear-to-pale eosinophilic material (mucin). H&E. C. Neoplastic cells show strong cytoplasmic reactivity to vimentin immunohistochemical staining. D. Myxomatous matrix shows strong reactivity to alcian blue histochemical staining, confirming the presence of mucin.

Figure 3.

A. A neoplastic nodule (bottom of image) is compressing and infiltrating surrounding pulmonary alveoli (top of image). H&E. B. Neoplastic areas (bottom of image) are compressing adjacent hepatocytes (top of image). H&E. C. Neoplastic lobules (margins on arrows) are effacing the tunica muscularis, infiltrating the submucosa, and approaching the mucosa (margin on arrowheads). H&E. D. Neoplastic cells within the liver exhibit moderate-to-marked cellular and nuclear pleomorphism. There are 3 bizarre mitotic figures within the field, 2 marked by arrowheads and a “sunburst” bizarre mitotic figure that is marked by an arrow. H&E.