Uterine or endometrial cancer forms in the tissue lining of the uterus, the pear-shaped organ where a fetus develops. In the most common type of uterine cancer, called endometrial adenocarcinoma, cancer begins in the cells that make and release mucus and other fluids. In uterine sarcomas, malignant cells form in the muscle of the uterus or in the uterine lining. Uterine sarcomas are much less common than endometrial cancer, accounting for fewer than five percent of all uterine cancers. However, they are much more aggressive and can spread quickly to other sites. For 2010, the National Cancer Institute estimated 43,470 new cases of uterine cancer and 7,950 deaths from the disease.