Choriocarcinoma is a very rare and aggressive type of testicular cancer that occurs mostly in adults.  These cancers are likely to spread rapidly to distant organs of the body, including the lungs, bone and brain.  Pure choriocarcinoma does not often occur in the testicles.  More often, choriocarcinoma cells are present with other types of nonseminoma germ cells in a mixed germ cell tumor.  This type of tumor increases blood levels of HCG.