Primary extraskeletal duodenal osteosarcoma with peritoneal sarcomatosis in a cat

This case report describes an extraskeletal intestinal osteosarcoma with sarcomatosis in an 11-year-old female neutered Domestic Longhair cat that presented with a 2-week history of hyporexia, lethargy, vomiting and polydipsia. Clinical examination identified a cranial abdominal mass and ultrasound examination revealed liver nodules and an irregularly shaped, partly mineralized mass around the proximal duodenum and pancreas. Cytological examination of the intestinal and liver lesions identified plump to fusiform to stellate cells with scattered foci of mineralization. Post-mortem examination identified an extraskeletal intestinal osteosarcoma originating in the wall of the duodenum with sarcomatosis on the serosal surface of the liver, gallbladder and omentum. Local infiltration into the pancreas, liver and diaphragm was also present. Increased awareness and reporting of sarcomatosis in cats are important for expanding the knowledge base, guiding clinical management and understanding its impact on survival in such rare cases. © 2025 The Author(s). Published by Elsevier Ltd.