Presentation
Headaches.
Patient Data
CT Brain
There is a hyperdense mass arising from the roof of the 4th ventricle causing hydrocephalus. Following contrast, there is a heterogeneous enhancement of this posterior fossa mass.
No other lesions are seen in the posterior fossa or the cerebral hemispheres.
Features are those of a tumor with high cellularity arising from the roof of the fourth ventricle, and medulloblastoma should be highly considered. Cerebellar metastasis, choroid plexus papilloma, ependymoma are much less likely differentials.
MR Brain
There is an enhancing mass apparently arising from the roof of the 4th ventricle, hyperintense on both T2 and FLAIR sequences, and exhibiting restricted diffusion on DWI sequence.
Case Discussion
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.
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