A malignant metastatic form of trophoblastic tumors. Unlike the hydatidiform mole, choriocarcinoma contains no chorionic villi but rather sheets of undifferentiated cytotrophoblasts and syncytiotrophoblasts (trophoblasts). It is characterized by the large amounts of chorionic gonadotropin produced. Tissue origins can be determined by dna analyses: placental (fetal) origin or non-placental origin (choriocarcinoma, non-gestational).