Uterus-like mass arising in the broad ligament: a metaplasia or mullerian duct anomaly?

A 46-year-old woman presented with a large abdominopelvic mass. Exploration revealed a large cystic mass that arose from the posterior leaf of the right broad ligament. The mass was lined by endometrial-like tissue and had an outer layer of smooth muscle that resembled myometrium. The inner wall was focally composed of a thick layer of intermixed smooth muscle and endometrial glands comparable to adenomyosis. This is the fourth reported case of "a uterus-like mass" and the first that is clearly extraneous to the ovary. We hypothesize that these masses may arise from subperitoneal mesenchymal cells that retain the ability to duplicate mullerian duct structures; other cases may represent mullerian duct fusion defects.