In horses, osteosarcoma is a rare tumor, with the majority of reported cases occurring in the head, and, more specifically, in the mandible of young horses. The following report documents 8 cases of equine osteosarcoma, the majority occurring in male horses aged 7 years or older with a lack of metastasis identified in any horse. Six arose in the maxilla or mandible and one in the proximal tibia. The predominant subtype was fibroblastic osteosarcoma with fewer osteoblastic type tumors. All had osteoid and most had a chondromucinous matrix. Surgical excision was attempted in the majority of cases. An inability to completely excise the tumor and progressive disease typically resulted in euthanasia. To the authors' knowledge, this case series also documents the first report of an equine extraosseous osteosarcoma within the subcutaneous tissue caudal to the shoulder. Surgical excision appears successful with no recurrence of disease 14 months later. Further investigations of equine osteosarcoma and various chemotherapeutic agents are warranted to present additional treatment options.
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