Uterine cancer is the most common cancer of a woman’s reproductive system. Uterine cancer begins when normal cells in the uterus change and grows uncontrollably, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can spread to other parts of the body. A benign tumor means the tumor will not spread. Noncancerous conditions of the uterus include fibroids, which are benign tumors in the muscle of the uterus. Another noncancerous condition is endometriosis, which describes the condition when endometrial tissue, typically lining the uterine cavity, is on the outside of the uterus or other organs. Endometrial hyperplasia, which is an increased number of cells in the uterine lining, can also happen and can be noncancerous.