An ill-defined mixed solid and cystic tumour centred on the cerebellar vermis and extending predominately into the left cerebellum. A second enhancing solid nodule in the left lateral cerebellum. The solid components show heterogeneous enhancement and diffusion restriction (-600 x 10^-6 mm^2/s). A single tiny focus of susceptibility artefact in the central portion of the lesion. No T2 flow voids around the lesion.
Mass effect on the fourth ventricle, tonsillar descent, with marked dilatation of the lateral and third ventricles, with periventricular high T2 /FLAIR signal compatible with subependymal oedema.
Conclusion:
Cerebellar tumour with mass effect causing obstructive hydrocephalus and tonsillar herniation. Medulloblastoma is the favoured diagnosis (most likely SHH subtype).