An extraskeletal osteosarcoma in the auricle of a Wistar Hannover rat.

An extraskeletal osteosarcoma was detected in the auricle of a 110-week-old female Wistar Hannover rat. Grossly, the tumor, measuring 15 mm in size, was observed in the subcutis as a solid and hard mass. Histologically, the majority of the mass comprised mature, compact bone. It was surrounded by neoplastic cells showing a variety of histologies, such as sarcoma, not otherwise specified, and myxosarcoma away from the bone-forming region. However, these different histological regions were considered to be components of a single bone tumor, based on the common expression of osterix and a similar mixture of constituent cells in each region. The tumor was diagnosed as an extraskeletal osteosarcoma because of the presence of infiltrative growth and abnormal mitosis and its development in the auricle without attachment to the skeleton. The present case is a rare histological type of an extraskeletal osteosarcoma with independent and different histological elements in rats.