Extraskeletal chondroblastic osteosarcoma of the subcutaneous tissue in a Maltese dog

A 10-year-old spayed female Maltese dog presented to a local animal hospital with a subcutaneous mass (4 x 3 x 3 cm) in the right shoulder region. The mass was well circumscribed, with soft tissue opacity and variable levels of mineralization, but with no bony involvement in radiography. The mass was surgically removed. Upon histological examination, the mass consisted of malignant mesenchymal cells that had produced a chondroid matrix and osteoid. Round to polygonal neoplastic cells with large nuclei showed moderate anisokaryosis and variable numbers of mitotic figures. The tumor cells were positive for vimentin and osteocalcin and negative for pancytokeratin, S100, and C-kit. On the basis of histopathologic and immunohistochemical features, the tumor was diagnosed as an extraskeletal chondroblastic osteosarcoma.