👉 An endothelioma is a type of cancer that occurs in the blood vessels of the heart, lungs, and other organs. It is a rare form of cancer that develops from abnormal blood vessels in these organs. In an endothelioma, cells grow uncontrollably and spread throughout the body. The cells can affect the heart, lungs, brain, or any other organ in the body. This type of cancer is usually treatable with surgery, radiation therapy, chemotherapy, and possibly a bone marrow