Ovarian cancer: A malignant tumor of the ovary, the egg sac in a female. Women who have a family history of ovarian cancer are at an increased risk of developing ovarian cancer. Hereditary ovarian cancer makes up a small percentage of all cases of ovarian cancer. Three hereditary patterns have been identified: ovarian cancer alone, ovarian and breast cancers, and ovarian and colon cancers. Ovarian cancer is difficult to detect early because there usually are no symptoms and the symptoms that do occur tend to be vague. Detection involves physical examination (including pelvic exam), ultrasound, X-ray tests, CA 125 test, and biopsy of the ovary. Most ovarian growths in women under age 30 are benign (noncancerous), fluid-filled cysts.