Subcutaneous extraskeletal chondroblastic osteosarcoma in a cat

CASE SUMMARY: An 11-year-old neutered male cat was presented with a fixed, subcutaneous mass in the left hindlimb. The neoplasm was surgically removed and determined to be a 2 × 2 × 9 cm mass that extended over the plantar surface of the left hindlimb from the tarsus to the phalanges. It was independent from the skeletal system but firmly attached to the adjacent connective tissue. Microscopically, the neoplasm was composed of highly proliferative mesenchymal neoplastic cells that formed both osseous and cartilaginous tissues with associated production of chondroid, osteoid and associated matrixes. This neoplasia was diagnosed as an extraskeletal chondroblastic osteosarcoma. Extraskeletal osteosarcomas, especially the chondroblastic subtype, are extremely rare in cats. Consequently, little is known concerning their course and prognosis. In this case, excision with wide margins appeared to be successful as, at the time of writing, 24 months after limbectomy, the cat is healthy with no evidence of recurrence or metastasis. RELEVANCE AND NOVEL INFORMATION: To our knowledge, this is the first report of an appendicular large extraskeletal chondroblastic osteosarcoma occurring in a domestic cat. As these neoplasms are rare, it should be considered as a less likely cause of soft tissue appendicular neoplasms in domestic cats.