Medulloblastomas are the most common pediatric posterior fossa tumor and account for 30-40% of such entities. They typically present as midline masses in the roof of the 4th ventricle with associated mass effect and hydrocephalus. 

The tumors, in general, tend to be extremely cellular and is an example of a small round blue cell tumor which results in predictable imaging features, such as hyperdense appearances on non-contrast CT and restricted diffusion (DWI/ADC). 

The vast majority (94%) of medulloblastomas arise in the cerebellum and the majority of these, from the vermis (75%). They tend to protrude into the fourth ventricle from its roof, and may even grow directly into the brainstem.

Although medulloblastomas project into the fourth ventricle, unlike ependymomas they do not usually extend into the basal cisterns.

As CSF seeding is common at presentation, imaging with the contrast of the whole neuraxis is recommended to identify drop metastases and leptomeningeal spread.