Mediastinal Extraskeletal Osteosarcoma in a Canine with Pulmonary and Cerebral Metastasis

Background: Extraskeletal osteosarcoma (EOS), a rare variant of osteosarcoma (OS), is a malignant neoplasm that develops in soft tissues without primary bone involvement. This study aims to describe a case of EOS with a mediastinal location in a canine.Case: A 10-year-old male, Uruguayan, Cimarron dog was presented to the Laboratório Regional de Diagnóstico, Faculdade de Veterinária, Universidade Federal de Pelotas (LRD/FV/UFPel) for necropsy. The dog had a history of submandibular swelling, progressive hind limb paralysis, muscle atrophy, and breathing difficulties. During necropsy, in the thoracic cavity, approximately 900 mL of serosanguinous exudate and a reddish-brown, bossed mediastinal mass measuring 15.0 cm in the longest axis were also noted. The lung exhibited multifocal to coalescent, white, firm nodules extending from the pleura to the parenchyma and measuring up to 4.5 cm in diameter. In the parietal and occipital region of the brain, a matte wine mass measuring 2.3 cm in the longest axis was observed. Fragments of the neoplastic mass, organs of the abdominal and thoracic cavities, and the brain were harvested and fixed in 10% buffered formalin. After 48 h, the samples were routinely processed, incorporated in paraffin, cut into 3 µm-thick sections, and stained using Hematoxylin and Eosin (HE). Selected sections of the neoplasm, the lung, and the brain were subjected to Von Kossa staining and immunohistochemical (IHC) analysis. For IHC, primary anti-cytokeratin monoclonal antibodies (clone AE1 / AE3, BioCare Medical) at a 1:100 dilution, vimentin (clone V9, BioCare Medical) at a 1:100 dilution, S100 Protein (clone 15E2E2, BioCare Medical) at a 1:100 dilution, and Ki67 (SP6 clone, BioCare Medical) at a 1:50 dilution were used. Immunostaining was visualized using 3-3' diaminabenzidine (DAB). Histological evaluation of the mediastinal mass, the pulmonary nodules, and the central nervous system among polygonal mesenchymal cells was conducted. Marked pleomorphism with euchromatic, rounded to oval nuclei, evident nucleoli, and poorly delimited eosinophilic cytoplasm. Neoplastic cells were arranged in nests and bundles with an invasive growth pattern. Osteoid and bone matrix formation as well as multinucleated giant cells of the osteoclast type were observed. The bone matrix was better evidenced in Von Kossa staining. IHC in all analyzed sections of the neoplastic cells showed positive immunostaining for Vimentin and Ki67. In the sections incubated w