Although fibroids are called tumors, these growths are usually composed of benign cells. When they reach sufficient sizes, fibroids can put pressure on other organs or distort the shape of the uterus. Studies have shown that it is rare for a fibroid to become cancerous.
Those that do, however, develop into the type of malignancy known as a sarcoma. U.S. health statistics show that less than 1% of fibroids become cancerous.