Brain tumour. Coloured magnetic resonance imaging (MRI) scans of sections through the head of a 23-year-old patient with a lesion (dark patches, upper centre-right; lower left and right) in the cerebellum indicative of the presence of a medulloblastoma. Medulloblastoma is a highly malignant (cancerous) primary brain tumor that originates in the cerebellum or posterior fossa regions of the brain. It is invasive and rapidly growing tumours and, unlike most brain tumours, spreads through the cerebrospinal fluid and frequently metastasizes to different locations in the brain and spine.