Medulloblastoma is a cancerous brain tumor that starts near the brainstem in the cerebellum. The tumor is fast growing and can spread to other areas of your brain and spinal cord. Medulloblastoma is more common in children than adults. PopulationWhile medulloblastoma is rare, it is the most common pediatric malignant (cancerous) brain tumor. Approximately 20% of childhood brain tumors are medulloblastomas. According to the American Cancer Society, about 500 children are diagnosed with medulloblastoma each year in the United States. There are four known subtypes of childhood medulloblastoma.