- A type of cancer that begins in cells that form sperm or eggs. There are several types of nonseminoma tumors, including embryonal carcinoma, malignant teratoma, choriocarcinoma, and yolk sac tumor. These tumors are usually made up of more than one type of cancer cell. Although nonseminomas occur most often in the testicles or ovaries, they can occur in other tissues, such as the brain, chest, or abdomen. This happens when cells that have the ability to form sperm or eggs are found in other parts of the body.