The most malignant and the second most common posterior fossa tumor, medulloblastoma is considered a primitive neuroectodermal tumor (PNET) and arises from the inferior medullary velum of the cerebellar vermis. The tumor invades the fourth ventricle and is typically a poorly defined mass. Lesions are hypercellular and are typically hyperdense on unenhanced CT, enhancing diffusely and homogeneously. On MRI, lesions also tend to be more homogeneous. CSF spread/seeding can occur both intracranially and as drop metastases to the intraspinal CSF.