Teratomas are malign germ cell tumors composed of two or more tissue layers. When there is specific organ differentiation they are called mature teratoma. They rarely grow aggressively. We report the case of a 29 year-old man with a diagnosis of gonadal germ cell tumor whose evolution was unfavorable owing to transformation into a different phenotype corresponding to a rhabdomyosarcoma. This phenomenon occurs through differential growth of a single histological component of the original tumor or transformation of a somatic lineage that becomes dominant. Transformed tumors such as the one herein described differ from most germ cell neoplasms regarding behavior, prognosis, and susceptibility to established treatments.
Keywords: germ cell tumor; malignant transformation; mature teratoma; rhabdomyosarcoma.