HCG is a tumor marker for:

Gestational choriocarcinoma is an extremely malignant tumor comprising of sheets of anaplastic cytotrophoblast cells and syncytiotrophoblast with prominent hemorrhage, necrosis, and vascular invasion. This is associated with high levels of Beta HCG. 2/3rd cases of gestational choriocarcinoma occur after term pregnancy and one third develop after a spontaneous aboion or pregnancy termination. Initially it invades the endometrium and myometrium and later undergo blood borne systemic metastasis. Ref: Hoffman B.L., Schorge J.O., Schaffer J.I., Halvorson L.M., Bradshaw K.D., Cunningham F.G., Calver L.E. (2012). Chapter 37. Gestational Trophoblastic Disease. In B.L. Hoffman, J.O. Schorge, J.I. Schaffer, L.M. Halvorson, K.D. Bradshaw, F.G. Cunningham, L.E. Calver (Eds), Williams Gynecology, 2e.