Doctors use many tests to diagnose a tumor and find out if it has metastasized (spread). Some tests may also determine which treatments may be the most effective. For most types of tumors, a biopsy or removal of the tumor with surgery is the only way to make a definitive diagnosis. If a biopsy or surgery to remove the tumor is not possible or medically safe, the doctor may suggest other tests that will help make a diagnosis. Imaging tests may be used to find out whether the tumor has metastasized. Your child's doctor may consider these factors when choosing a diagnostic test:

• Age and medical condition
• Type of tumor suspected
• Severity of symptoms
• Previous test results

In addition to a physical examination, the following tests may be used to diagnose medulloblastoma:

Biopsy [2]. A biopsy is the removal of a small amount of tissue for examination under a microscope. Other tests can suggest that cancer is present, but looking at the tumor tissue removed during a biopsy or surgery (see below) is the only way to make a definite diagnosis. The sample removed during the biopsy is analyzed by a pathologist (a doctor who specializes in interpreting laboratory tests and evaluating cells, tissues, and organs to diagnose disease).

Surgical removal of the tumor [3]. During surgery, a neurosurgeon (a doctor who specializes in removing a tumor from the brain or spine with surgery) removes as much of the tumor as possible for examination under a microscope (see the Treatment [4] section). The sample removed during surgery is then analyzed by the pathologist.

Computed tomography (CT or CAT) scan [5]. A CT scan creates a three-dimensional picture of the inside of the body with an x-ray machine. A computer then combines these images into a detailed, cross-sectional view that shows any abnormalities or tumors. Sometimes, a contrast medium (a special dye) is injected into a vein to provide better detail.

Magnetic resonance imaging (MRI) [6]. An MRI uses magnetic fields, not x-rays, to produce detailed images of the body. A contrast medium may be injected into a patient's vein to create a clearer picture.

Lumbar puncture (spinal tap). A lumbar puncture is a procedure in which a doctor uses a needle to take a sample of cerebral spinal fluid (CSF) to look for cancer cells, blood, or tumor markers (substances found in higher than normal amounts in the blood, urine, or body tissues of people with certain kinds of cancer). CSF is the fluid that flows around the brain and the spinal cord. The child is given an anesthetic that numbs the lower back before the procedure. For patients with medulloblastoma, doctors usually perform a lumbar puncture after the tumor is removed through surgery (see the Treatment [4] section).

Learn more about what to expect when having common tests, procedures, and scans [7].

After these diagnostic tests are done, your child's doctor will review all of the results with you. If the diagnosis is a tumor, these results also help the doctor describe the tumor; this is called staging [8]. Learn more about the first steps to take after a diagnosis [9].