The ovaries, which produce eggs, are positioned on each side of the uterus. Ovaries are the main source of a woman's female hormones, estrogen and progesterone. Ovarian cancer is a disease in which cancerous cells form in the tissues of the ovary. Ovarian epithelial cancer is the most common type of ovarian cancer, in which malignant cells form in the tissue covering the ovary. Primary peritoneal cancer arises when similar changes occur in the cells lining the abdomen (the peritoneum).