A type of cancer that begins in germ cells (cells that form sperm or eggs). Seminomas occur most often in the testicle, but they may also occur in other areas of the body, such as the brain, chest, or abdomen. Seminomas tend to grow and spread slowly. Some seminomas make a hormone called beta-human chorionic gonadotropin (beta-HCG) that may cause signs and symptoms of disease. Seminomas usually occur in young men.