CSF levels of AFP are a useful aid in the diagnosis of primary central nervous system (CNS) germ cell tumors (GCTs). These account for approximately 4 % of primary brain tumors in children. AFP elevation in CSF occurs in approximately 70 % of these tumors. Concurrent testing of beta-HCG is recommended to improve the sensitivity for detecting these CNS tumors. CSF levels of AFP will also be elevated with brain metastases of testicular cancer.