Cancer of the chorion. Coloured 3-D ultrasound scan of a tumour (centre) in the uterus, caused by a cancer of trophoblast cells of the placenta, called choriocarcinoma. Trophoblast cells form part of the chorion, the membrane that surrounds a developing embryo. The chorion connects to maternal tissue to form the placenta. Choriocarcinoma may occur when an embryo fails to develop, but the placental tissue continues to proliferate. It is treated by surgical removal, radiotherapy and chemotherapy. Ultrasound is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The echoes form an image of an internal structure.