Cancer occurs when cells undergo a change called a malignant transformation. They begin to grow and multiply without normal controls. As the cells grow and multiply, they form masses called tumors. Cancer can also spread or metastasize from their site of origin into the other tissues. Cancer is dangerous both because of its growth and spread. Cancer growths overwhelm healthy cells by taking their space and the oxygen and nutrients they need to survive and function.Ovarian cancer occurs when a tumor forms in one or both of a woman's ovaries. The ovaries are a pair of small organs that produce and release ova, or human eggs. The ovaries also produce important hormones such as estrogen and progesterone. They are located in the lower abdomen (pelvis), on either side of the womb (uterus). Ova released by the ovaries travel through the fallopian tubes to the uterus, where they may or may not be fertilized by the male sperm.Cancerous tumors are malignant. This means they can spread to other tissues and organs. Not all tumors, however, are malignant. The ovaries can develop benign tumors, as well as malignant tumors or cancers.In the process called metastasis, malignant tumors may encroach on and invade neighboring organs or lymph nodes, or they may enter the bloodstream and spread to remote organs such as the liver or lungs. The presence of metastases or metastatic tumors is an ominous finding noted in the more advanced stages of cancer of the ovary.The type of cell that originated the abnormal growth determines the class of the ovarian tumors.Epithelial tumors: These tumors arise from a layer of cells that line the ovary called the germinal epithelium. A majority of all ovarian cancers are epithelial. These are most common in women who have been through menopause (aged 45-70 years). These epithelial tumors are rarely fount without at least some evidence of spread. Chemotherapy is used in addition to surgery to treat these cancers.Stromal tumors: Stromal tumors develop from connective-tissue cells that help form the structure of the ovary and produce hormones. Usually, only one ovary is involved. These account for 5-10% of ovarian cancers. These tumors typically occur in women aged 40-60 years. Often, surgical removal of the tumor is the only treatment needed. If the tumor has spread, though, the woman needs chemotherapy.Germ cell tumors: Tumors that arise from germ cells (cells that produce the egg) account for about 15% of all ovarian cancers. These tumors develop most often in young women (including teenaged girls). Although 90% of women with this type of cancer are successfully treated, many become permanently infertile.Metastatic tumors: Only 5% of ovarian cancers have spread from other sites to the ovary. The most common sites from which they spread are the colon, breast, stomach, and pancreas.Within these main classes are many different subtypes of tumors.Noncancerous (benign) ovarian masses include abscesses or infections, fibroids, cysts, polycystic ovaries, endometriosis-related masses, ectopic pregnancies, and others.Of markedly enlarged ovarian masses (>4 cm) found in women who are still menstruating (have not been through menopause), about 20% are cancerous.Of markedly enlarged masses found in women who have been through menopause, about 45%-50% are cancerous.The incidence of ovarian cancer varies greatly. Globally, Scandinavia, Israel, and North America have the highest rates. Developing countries and Japan have the lowest rates.Some 14,180 women in the U.S. die each year from ovarian cancer.The five-year survival rate is greater than 75% if diagnosis of the cancer occurs before it has spread to other organs. However, the five-year survival rate drops to 20% when the tumor has spread to the upper abdomen.In the United States, about one in 56 women develops cancer of the ovary. About 21,290 new cases in the U.S. are diagnosed each year.