Hemangiosarcoma is an aggressive cancer arising from the cells that line blood vessels (vascular endothelium). In the spleen, it forms blood-filled tumours that can rupture without warning, causing life-threatening internal bleeding (haemoabdomen) — this emergency presentation is often the first sign of the disease. Stage I–II means the tumour is confined to the spleen without visible distant spread, though microscopic metastatic disease is presumed present in the vast majority of cases at diagnosis. Large and giant breeds, particularly Golden Retrievers and German Shepherd Dogs, are at highest risk. Even after surgical removal of the spleen, the cancer has almost always begun spreading microscopically, which is why follow-up chemotherapy is strongly recommended to delay the growth of these tiny secondary deposits.