In this case, what entities should be considered?
Ependymoma, choroid plexus papilloma, pilocytic astrocytoma, haemangioblastoma and possibly medulloblastoma.
Which is most likely? What is the one atypical feature for this entity?
The mass has appearances most in keeping with a choroid plexus papilloma: a very vividly enhancing lobulated mass within the ventricle with associated hydrocephalus. Slightly atypical is the location; unlike most paediatric brain tumours, in children choroid plexus papillomas are more frequently found in the trigone of the lateral ventricles.
What features are good for haemangioblastoma? What is against the diagnosis?
The location, vivid enhancement, flow voids and associated cystic component are all seen in haemangioblastomas. The age is atypical. Peak incidence is around 30 - 60 years of age, but earlier in patients with von Hippel Lindau (vHL) - in which case they are usually multiple.
What about ependymoma?
Although the age group and location are good, ependymomas tend to be less 'frond-like' and enhancement, although prominent, less vivid and homogeneous than choroid plexus papillomas. Ependymomas often have associated oedema in the adjacent brain; in this case there is only a little next to the cystic component superiorly on the left.
Brain MRI demonstrates a multilobulated fourth ventricular mass with cystic changes. The mass is isointense to gray matter on T1WIs, iso to hypointense on T2WIs and demonstrates avid enhancement on post contrast sequences.