Temporary embolic agents block up blood vessels a long enough time (days to weeks) for the body to heal the underlying health problem. For example, after a car accident, Gelfoam embolization can stop internal bleeding. After several days the body begins to heal the injury. By the time the Gelfoam dissolves, the healing process at the site of injury is far enough along to prevent rebleeding. Permanent embolic agents mechanically plug-up blood vessels and cause scar tissue to form in the vessel that doesn't go away. This is important in treating arteriovenous malformation and tumors; in these cases, if the embolic agent dissolved, the problem could recur. In all embolization procedures, the radiologist will inject contrast material into the vessel to measure the progress of the procedure and to decide when embolization is complete.
Uterine fibroids and other types of tumors, like all tissues, depend on a steady supply of oxygen and nutrients that are carried by the arteries that feed them. Once the supply is cut off by embolization, the tissue starts to break down and, in successful cases, the tumor grows smaller or occasionally is eliminated.