Canine extraskeletal osteosarcomas are extremely rare tumors. Over a period of 25 years at the Animal Medical Center, approximately 1,000 cases of skeletal osteosarcomas have been diagnosed. During the same period 11 cases of extraskeletal osteosarcomas and three extraskeletal chondrosarcomas were diagnosed. Tumors of the mammary gland were excluded. Extraskeletal osteosarcomas were found in the adrenal gland, eye, gastric ligament, ileum, kidney, liver, spleen, testicle, and vagina. The chondrosarcomas were found in the mitral valves, lungs, and omentum. The mean age of the dogs with extraskeletal osteosarcoma was 11 years, and the mean age of the dogs with extraskeletal chondrosarcoma was 14 years. The sizes of the tumors ranged from 3 cubic centimeters to 8,315 cubic centimeters. Osteoblastic osteosarcomas were the most common histologic type (7/11, 63.6%); there was a single case of each of the following: fibroblastic, fibrous histiocytic, chondroblastic, and mixed osteo-chondroblastic osteosarcoma. Two of the dogs with chondrosarcomas had mesenchymal chondrosarcomas involving the lungs and omentum. The remaining dog had a regular chondrosarcoma involving the mitral valve. Distant metastases were present in seven of 11 dogs with extraskeletal osteosarcoma and in none of the dogs with chondrosarcoma. In contrast to human beings, in which most extraskeletal osteosarcomas occur in the soft tissues and the extremities, most canine extraskeletal osteosarcomas develop in the visceral organs. © 1990, American College of Veterinary Pathologists. All rights reserved.
