Ovarian cancer refers to a cancerous tumor that begins in a woman's ovaries but may spread, or metastasize, to other parts of the body.

Although ovarian cancer affects far fewer women than many other cancers, it is troublesome because it is difficult to detect in its early stages, when treatment can be most effective. For this reason it is especially important for women to identify whether they are at particularly high risk, so they can have more rigorous screening than is available to the general public.

The ovaries are a pair of almond-sized organs located on either side of a woman's uterus. They produce eggs, as well as the female hormones estrogen and progesterone, which regulate the menstrual cycle and pregnancy.

The ovaries are composed of layers of cells. The inner layer, or germ layer, produces eggs. The outer covering is referred to as the epithelial layer.

In all cancers, cells in the human body change and experience out-of-control growth. In the case of ovarian cancer, this can occur in the epithelial layer, the germ layer, or the supportive tissues that surround the ovaries. By far the most common type of ovarian tumors are those that begin in the epithelial layer: Accounting for approximately 90 percent of all ovarian cancers, this type of malignancy is called ovarian epithelial cancer.

Ovarian cancers metastasize in a couple of ways: In a process called shedding, ovarian cancer cells can break away from the ovary and spread to other tissues and organs. In the process, new tumors tend to form on the peritoneum (the large membrane that lines the abdomen) and on the diaphragm (the thin breathing muscle that separates the chest from the abdomen).

Ovarian cancer cells can also enter the bloodstream or lymphatic system (the tissues and organs that produce and store cells that fight infection and disease), where they can travel to other parts of the body and form new tumors.