Uterine tumors are rare occurrences, usually affecting middle-aged to older female cats that have not been spayed. These tumors arise from the uterine smooth muscle and epithelial tissues -- the tissues that line the internal organs and cavities. Cats usually develop malignant metastatic (aggressive and spreading) uterine tumors called adenocarcinomas, tumors that are derived from the glands. These types of tumors are called Müllerian tumors, since the uterus is derived from the Müllerian ducts in the embryo.