ovarian cancer : Causes

An ovarian tumor is a growth of abnormal cells that may be either noncancerous (benign) or cancerous (malignant). Although benign tumors are made up of abnormal cells, these cells don't spread to other body tissues (metastasize). Ovarian cancer cells metastasize in one of two ways. Most often, they spread directly to other organs in the pelvis and abdomen. Rarely, they can spread through your bloodstream or lymph nodes to other parts of your body.

The causes of ovarian cancer remain unknown. Some researchers believe it has to do with the tissue-repair process that follows the monthly release of an egg through a tiny tear in an ovarian follicle (ovulation) during a woman's reproductive years. The formation and division of new cells at the rupture site may set up a situation in which genetic errors can occur. Others propose that the increased hormone levels before and during ovulation may stimulate the growth of abnormal cells.

Three basic types of ovarian tumors exist, designated by where they form in the ovary. They include:

Epithelial tumors. About 90 percent of ovarian cancers develop in the epithelium, the thin layer of tissue that covers the ovaries. This form of ovarian cancer generally occurs in postmenopausal women.
Germ cell tumors. These tumors occur in the egg-producing cells of the ovary and generally occur in younger women.
Stromal tumors. These tumors develop in the estrogen- and progesterone-producing tissue that holds the ovary together.

http://www.mayoclinic.com/health/ovarian-cancer/DS00293/DSECTION=3