Extraskeletal osteosarcoma in two dogs.

Extraskeletal osteosarcoma (ESOS) of the spleen and jejunum was diagnosed in 2 dogs. As an extremely uncommon type of tumor that has proven difficult to treat, ESOS is associated with high rate of local recurrence and metastatic disease. Extraskeletal osteosarcoma principally affects older dogs, has no apparent breed predilection, and may develop more frequently in males. The cause of ESOS is unknown, but may involve malignant metaplasia of pluripotential mesenchymal cells into osteoblasts. Macroscopically, ESOS usually is observed as a hard mass and may appear similar to calcified hematoma or myositis ossificans. The classic radiographic appearance of ESOS is a soft tissue mass with focal mineralization and without adjacent bone involvement.