Primary extraskeletal osteosarcoma of the omentum in a young dog

Abstract

Extraskeletal osteosarcoma (EOSA) is a rare and aggressive mesenchymal neoplasm in dogs, primarily affecting older animals, with the spleen being the most common site of involvement. This report describes a rare case of primary EOSA originating in the omentum of a 1-year-and-5-month-old spayed female dog, highlighting the role of immunohistochemistry, specifically the positive expression of vimentin and RUNX2, in confirming the tumor’s mesenchymal and osteoblastic origin. The tumor was inoperable, and treatment consisted of medical management with toceranib phosphate and losartan, with the addition of zoledronic acid following the detection of bone metastasis. The patient exhibited temporary clinical improvement for 90 days, after which disease progression was noted, including the development of abdominal effusion (1.2 liters drained). The patient survived for 180 days following diagnosis, without adverse effects related to the treatment. Necropsy confirmed widespread metastases from fibroblastic osteosarcoma, including the skin, peritoneum, lungs, heart, liver, lymph nodes, and brain, with cerebral hemorrhage as the likely cause of sudden death. Histopathological evaluation revealed extensive areas of osteoid matrix and proliferating mesenchymal cells in a disorganized arrangement, with marked vascular invasion by neoplastic emboli, as well as multifocal hemorrhage and tumor necrosis. This case illustrates a rare presentation of EOSA in a young dog and emphasizes the importance of including neoplasia in the differential diagnosis of abdominal masses, regardless of patient age. It also highlights the diagnostic value of immunohistochemical markers, particularly RUNX2, in distinguishing EOSA from other mesenchymal or reactive proliferations.