TeratomaA teratoma is a type of tumour that derives from germ cells. Teratomas usually start from cells in the testes in men, the ovaries in women and in the sacrum in children. A teratoma is not a cancer. Teratomas often contain well-differentiated cells which can result in tissues growing in a teratoma which are quite different to the surrounding tissue - ovarian teratomas have been known to grow hair and teeth. Testicular teratomas are generally less well-differentiated, and have a worse prognosis (chances of recovery are not as high). A teratoma is also called a dermoid cyst.