What are the three main differentials?
Ependymoma, choroid plexus papilloma, and medulloblastoma.
What do you think is the most likely diagnosis? Why?
Ependymoma. The extrusion through the foramen of Luschka is characteristic ("plastic ependymoma"). Medulloblastomas tend to present in children, and usually demonstrate some restricted diffusion (small round blue cell tumours). Choroid plexus papillomas usually enhance even more brightly, and are usually associated with hydrocephalus.
MRI of the brain demonstrates a large heterogeneous mass centred in then fourth ventricle and herniating out of the left foramen of Luschka. It is mostly isointense to the adjacent cerebellum on T1 weighted images, hyperintense on T2 (with areas of higher signal / cystic change), and without significant restricted diffusion. Post contrast images demonstrate vivid but heterogeneous contrast enhancement.
There is no surrounding oedema, and although the fourth ventricle is effaced there are no features to suggest significant hydrocephalus on the images provided.