Teratomas are germ cell tumors that when seen under the microscope, look like each of the three layers of a developing embryo: endoderm (inner layer), mesoderm (middle layer) and ectoderm (outer layer).  There are three types of Teratoma Tumors.   

Mature teratomas are tumors formed by cells similar to cells of adult tissues.  They are generally benign and rarely spread to nearby tissues and distant parts of the body.  These can usually be cured with surgery.

Immature teratomas are less well-developed cancers with cells that look like those of an early embryo.  Unlike mature teratomas, this type is more likely to grow and invade surrounding tissues and to metastasize outside of the testicle.  This type can also recur years after treatment. 

Teratoma with malignant transformation is a very rare cancer.  These cancers have some areas that look like mature teratomas but have other areas where the cells have become a different type of cancer that develops outside of the testicle, in tissues such as muscles, glands of the lungs or intestines, or the brain.