Aggressive, undifferentiated sarcoma with widespread metastasis in a six-month-old Neopolitan mastiff.

A six-month-old Neopolitan mastiff presented for a rapidly growing cervical mass. Undifferentiated sarcoma was diagnosed at post mortem based on histopathology and immunohistochemistry. Metastases to mediastinum, pleura, lungs, liver, kidneys, omentum, mesentery, and multiple lymph nodes were present. Soft-tissue sarcomas are reported infrequently in children and young dogs. The cell of origin often is difficult to determine due to poor differentiation and rapid growth of these neoplasms.