Which diagnosis do you now favour? Why?
Medulloblastoma is by far the most likely. The mass restricts somewhat on DWI, does not strongly enhance and is located in the midline appearing to arrise form the vermis.
When found in adults, what do medulloblastomas more frequently demonstrate?
Unusual morphology and location. More frequently not midline.
What features, not present in this case, would make you favour an ependymoma?
Arising from the floor of the fourth ventricle, protruding out of the foramen of Luschka into the basal cisterns, more heterogeneous, more often calcified.
MRI confirms the presence of a midline lesion, which appears to arise from the vermis. The lesion is of low signal intensity on T1 and intermediate signal intensity on T2 weighted images. High signal intensity is noted on DWI. Heterogeneous enhancement is seen on post contrast sequences.
Acute hydrocephalus with transependymal oedema is seen as well.