Medulloblastoma is a brain cancer that usually occurs in children and young adults. Like all cancers, it is caused by the unbridled growth of abnormal cells that continue dividing without control. Over a period of time, these cells form a tumor or lump in the brain that keeps getting bigger. Medulloblastoma can spread to nearby parts of the brain and spinal cord, but not usually to other parts of the body. Brain tumors like medulloblastoma press on normal parts of the brain causing problems in brain function. This pressure on the brain causes symptoms such as headache, loss of balance, nausea, vomiting, and sleepiness, which are often worst in the morning.

In addition to pressure directly on the brain, medulloblastoma grows in a space that gets blocked off as the tumor gets larger. Medulloblastoma is most common in young children. The typical age at the time of diagnosis is around 7-8 years old, but it can occur in younger children as well as in adults. The tumor appears most often in the part of the brain where the cerebellum, fourth ventricle and brain stem meet. Medulloblastoma starts in this area but easily invades nearby tissue or spreads (metastasizes) to other parts of the brain and spinal cord. As many as 30% of patients have disease spread at the time the tumor is first diagnosed. But less than 5% of patients have tumor spread anywhere else in the body. If it spreads outside the brain and spinal cord, it usually goes to the bones.