A rare type of cancer that begins in germ cells (cells that form sperm or eggs). Yolk sac tumors occur most often in the ovary or testicle, but they may also occur in other areas of the body, such as the chest, abdomen, or brain. They tend to grow quickly and spread to other parts of the body if not treated. Yolk sac tumors are the most common malignant germ cell tumor in children. Also called endodermal sinus tumor.