Mediastinal Extraskeletal Osteosarcoma in a Canine with Pulmonary and Cerebral Metastasis
Authors: Alberti Taina dos Santos , Zamboni Rosimeri , Venancio Fabiano da Rosa , Brunner Carolina Buss , Raffi Margarida Buss , Schild Ana Lucia , Sallis Eliza Simone Viégas 
Published: June 2021

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 with anti-cytokeratin and S100 protein antibodies showed no presence of neoplastic cells.Discussion: The diagnosis of EOS was based on the absence of primary bone lesions during microscopic necroscopy and on the exclusion of other histogenetic origins using IHC. The absence of primary bone lesions was the main attribute that differentiates EOS from other variants osteosarcomas, such as central/medullary and surface OS (periosteal and paraosteal) most frequently in canine species. The origin of EOS is uncertain. However, its occurrence has been originated with in pluripotent cells or previous injuries, such as retention of surgical sponges and vaccination sites. In this case, since the animal had no clinical history of injuries or surgical procedures that could induce the formation of a mediastinal neoplasm, the probable origin of the neoplasm was pluripotent cells. In dogs, EOS occurs mainly in the mammary glands, digestive system, liver, spleen, and subcutaneous tissue. Furthermore, the mediastinal location observed in this study was described only in goats. The clinical signs are nonspecific and varies with the location of the tumor, as observed in the present report. EOS may also present mediastinal location in the canine species. Necropsy, histopathological examination, and IHC were essential to establish the diagnosis of this OS variant.