Parosteal fibrous maxillary osteosarcoma in a horse: a case report

Osteosarcoma is the most frequent malignant bone tumor in domestic animals and humans, representing 80-85% of malignant bone tumors in dogs [9, 10] and about 70-75% in cats. Only a few cases of osteosarcoma have been reported in horses with the majority in the mandible of young horses [3]. Maxillary osteosarcoma causes disruption of the bones with subsequent disruption of the dental arcade and interference with mastication [3, 5]. We describe a case of primitive parosteal fibrous maxillary osteosarcoma in a 16–year old Anglo- Arabian horse, hospitalized first for a clinical diagnosis of sinusitis. This case is also unusual in that generally maxillary fibrous osteosarcomas are low grade malignancies with minimal potential to metastatize, yet in this case the tumor had already spread to a regional lymph node by the time the horse was presented for examination, confirming the unpredictability of osteosarcoma.