This is a medical question of type : exams and tests 
 How to diagnose Childhood Central Nervous System Germ Cell Tumors ? 
 Imaging studies and tests are used to detect (find) and diagnose childhood CNS germ cell tumors. The following tests and procedures may be used:         -   Physical exam and history : An exam of the body to check general signs of health, including checking for signs of disease, such as lumps or anything else that seems unusual. A history of the patient's health habits and past illnesses and treatments will also be taken.     -   Neurological exam : A series of questions and tests to check the brain, spinal cord, and nerve function. The exam checks a persons mental status, coordination, and ability to walk normally, and how well the muscles, senses, and reflexes work. This may also be called a neuro exam or a neurologic exam.    -  Visual field exam: An exam to check a persons field of vision (the total area in which objects can be seen). This test measures both central vision (how much a person can see when looking straight ahead) and peripheral vision (how much a person can see in all other directions while staring straight ahead). The eyes are tested one at a time. The eye not being tested is covered.    -   MRI (magnetic resonance imaging) with gadolinium : A procedure that uses a magnet, radio waves, and a computer to make a series of detailed pictures of areas inside the brain and spinal cord. A substance called gadolinium is injected into a vein. The gadolinium collects around the cancer cells so they show up brighter in the picture. This procedure is also called nuclear magnetic resonance imaging (NMRI).     -   Lumbar puncture : A procedure used to collect cerebrospinal fluid (CSF) from the spinal column. This is done by placing a needle between two bones in the spine and into the CSF around the spinal cord and removing a sample of the fluid. The sample of CSF is checked under a microscope for signs of tumor cells. The sample may also be checked for the amounts of protein and glucose. A higher than normal amount of protein or lower than normal amount of glucose may be a sign of a tumor. This procedure is also called an LP or spinal tap.    -   Tumor marker tests : A procedure in which a sample of blood or cerebrospinal fluid (CSF) is checked to measure the amounts of certain substances released into the blood and CSF by organs, tissues, or tumor cells in the body. Certain substances are linked to specific types of cancer when found in increased levels in the blood. These are called tumor markers. The following tumor markers are used to diagnose some CNS germ cell tumors:               -  Alpha-fetoprotein (AFP).      -  Beta-human chorionic gonadotropin (-hCG).              -   Blood chemistry studies : A procedure in which a blood sample is checked to measure the amounts of certain substances released into the blood by organs and tissues in the body. An unusual (higher- or lower-than-normal) amount of a substance can be a sign of disease.    -  Blood hormone studies: A procedure in which a blood sample is checked to measure the amounts of certain hormones released into the blood by organs and tissues in the body. An unusual (higher- or lower-than-normal) amount of a substance can be a sign of disease in the organ or tissue that makes it. The blood will be checked for the levels of hormones made by the pituitary gland and other glands.
                    A biopsy may be done to be sure of the diagnosis of CNS germ cell tumor. If doctors think your child may have a CNS germ cell tumor, a biopsy may be done. For brain tumors, the biopsy is done by removing part of the skull and using a needle to remove a sample of tissue. Sometimes, a needle guided by a computer is used to remove the tissue sample. A pathologist views the tissue under a microscope to look for cancer cells. If cancer cells are found, the doctor may remove as much tumor as safely possible during the same surgery. The piece of skull is usually put back in place after the procedure.    Sometimes the diagnosis can be made based on the results of imaging and tumor marker tests and a biopsy is not needed.     The following test may be done on the sample of tissue that is removed:         -   Immunohistochemistry : A test that uses antibodies to check for certain antigens in a sample of tissue. The antibody is usually linked to a radioactive substance or a dye that causes the tissue to light up under a microscope. This type of test may be used to tell the difference between different types of brain tumors.