Extraskeletal osteosarcoma with cystic appearance in an aged Sprague-Dawley rat.

Extraskeletal osteosarcoma, arising spontaneously from the subcutis of the left abdomen and having a cystic appearance, was found in an untreated male Sprague-Dawley rat during a carcinogenicity study. At 76 weeks of age, the tumor mass had grown to 50 x 110 x 140 mm, and the animal exhibited severe anemia related to the complication of ulceration with hemorrhage. The tumor displayed irregular ossification at the walls of a cyst-like space filled with much yellowish fluid and necrotic cellular debris. Histopathologically, the tumor consisted of sheets of large, plump osteoblast-like cells, which produced broad irregular trabeculae of osteoid and calcified osseous tissue. Since extraskeletal osteosarcoma with a cystic appearance, has not been reported in animals, except for the telangiectatic type, our case shows an extremely rare type.