Primary Hepatic Hemangiosarcoma With Hemoperitoneum in a Dog: Clinical Course and Therapeutic Outcome.

A 14-year-old neutered male mixed-breed dog presented with hemoperitoneum secondary to rupture of a primary hepatic mass. Following stabilization with whole blood transfusion, a hepatic lobectomy was performed. Histopathological examination and CD31 immunolabeling confirmed a diagnosis of hemangiosarcoma. Two weeks postoperatively, multiple intra-abdominal metastases were identified. The multimodal therapeutic approach included one attempted VAC cycle (discontinued due to Grade 4 neutropenia), five cycles of single-agent doxorubicin, and subsequent administration of toceranib with dose reduction for anemia. The dog survived 201 days after diagnosis. Postmortem extended immunohistochemistry demonstrated heterogeneous receptor tyrosine kinase (RTK) expression: EGFR and VEGFR positivity, strong membranous HER2 labeling in approximately 60% of tumor cells, low KIT expression, and negative PDGFR. This case underscores the aggressive metastatic nature of primary hepatic hemangiosarcoma following rupture and highlights practical considerations for systemic therapy selection and adverse event monitoring in clinical practice.