What is the differential diagnosis?
In this age group a medulloblastoma should be considered probably most likely. A haemorrhagic metastasis (e.g. melanoma) should also be considered especially if there is a prior history of malignancy. Other primary tumours (e.g. haemangioblastoma, pilocytic astrocytoma etc...) are also in the mix but less likely.
A 3 x 4 cm irregular shape lesion is identified involving the left cerebellum and extending across mid-line. It is difficult to be certain as to whether the mass originated intra- or extra-axially. It certainly has an intraxial component with extensive oedema, but also has abuts the dura over a large area.
The lesion is heterogeneous on T1 with areas of low signal on T2 suggesting haemorrhage. Extensive high T2 surrounds the lesion in keeping with oedema. No restricted diffusion on DWI.
Following administration of contrast the lesion has a well defined enhancing edged but hypointense centre suggesting necrotic centre.
The 4th ventricle is distorted with evidence of obstructive hydrocephalus and tonsilar herniation.