Endometrial or uterine cancer can start in different parts of the uterus. Most uterine cancers start in the endometrium (the inner lining of the uterus). This is called endometrial cancer. Most endometrial cancers are adenocarcinomas (cancers that begin in cells that make mucus and other fluids). Uterine sarcoma is an uncommon form of uterine cancer that forms in the muscle and tissue that support the uterus. See Endometrial Cancer for signs, symptoms and treatments.