A uterine sarcoma or leiomyosarcoma is a cancerous tumor that arises from the same smooth muscle cells as fibroids, and in very rare instances, cancer cells can hide within fibroids and are very hard to detect using any type of imaging tests. Cancerous tumors are more often discovered during procedures being performed to remove fibroids. The removed specimens are studied under a microscope and examined for identifying characteristics of sarcoma.