Hemangiosarcoma is a type of cancer that originates in the blood vessels, commonly affecting dogs. It is an aggressive tumor that can occur in various organs, including the spleen, liver, and heart. Symptoms may include lethargy, weakness, and sudden collapse due to internal bleeding. Diagnosis often involves imaging techniques like ultrasound or X-rays, along with biopsies. Treatment options typically include surgery to remove the tumor and chemotherapy to manage the disease. Unfortunately, the prognosis for hemangiosarcoma is generally poor, as it tends to spread quickly and is often diagnosed at an advanced stage.