A 17 year old girl in the 4th month of pregnancy presents with vaginal bleeding, accompanied by passage of grapelike tissue fragments. Plasma human chorionic gonadotropin (hCG) is very high.Following ultrasound evaluation, dilatation and curettage is performed. Histologic examination shows diffuse trophoblastic proliferation and edema in all chorionic villi. Cytogenetic studies demonstrate 46, XY karyotype. Clinical studies show that this condition progresses to choriocarcinoma at a rate of?