Hemangiosarcoma (HSA) is a highly aggressive and malignant vascular tumor occurring most commonly in dogs. Dogs with splenic hemangiosarcoma have a poor prognosis, with fewer than 20% of dogs surviving 9 months with surgery and adjuvant doxorubicin chemotherapy. Paclitaxel is one of the most effective anticancer agents used in human cancer today, with potent activity against a wide variety of tumors. Paclitaxel interferes with normal microtubule function during cell division and induces cell cycle arrest. In addition, several studies have shown that paclitaxel has antiangiogenic properties. Paclitaxel targets both the tumor stroma and the vasculature, and it has been shown to induce tumor control in human patients with soft tissue angiosarcomas. The conventional formulation of paclitaxel (Taxol) has been associated with acute adverse events in dogs, which has largely precluded the use of paclitaxel as a chemotherapeutic in veterinary oncology. The investigational drug to be used in this study, Paccal Vet, uses a proprietary formulation based on retinol derivatives (XR-17), which encapsulate paclitaxel to form nanosized micelles. Previous studies with Paccal Vet have demonstrated a tolerability profile that is generally similar to other chemotherapy drugs used in veterinary oncology. Moreover, anti-tumor activity in selected cancers in dogs, including mast cell tumors, squamous cell carcinoma, and mammary cancer has also been reported for Paccal Vet. These findings suggest that Paccal Vet may be a promising treatment option for dogs with hemangiosarcoma. It is important to note that none of the chemotherapeutic agents commonly used in veterinary oncology clinics for the treatment of hemangiosarcoma have been approved for veterinary use.