Centred within the fourth ventricle and arising either from within the ventricle or from the ventral aspect of the vermis is a 3 cm diameter mass which appears moderately hyperintense on T2, intermediate to low signal on T1, and demonstrates heterogeneous contrast enhancement. No calcification. The mass demonstrates moderate restricted diffusion. High T2 signal (oedema) extends into the parenchyma of the vermis, but not into the pons.
It is surrounded by a number of small peritumoural cysts particularly posterolaterally. Prominent right PICA passes through the mass, best seen on axial T2 weighted imaging.
The ventricles are prominent with a mild degree of transependymal oedema consistent with a degree of hydrocephalus.
The infundibulum of the pituitary gland appears somewhat thickened and vividly enhancing, raising the possibility of abnormal nodular enhancement of this location.
The remainder of the brain is unremarkable with no intra or extra-axial abnormality.
Conclusion
Despite the patient's age, the location and apparent high cellularity (diffusion restriction) favours the diagnosis of a medulloblastoma.