A mixed solid and cystic tumour is present, arising from the left side of the midbrain, with the cyst projecting superiorly displacing the left thalamus and third ventricle, and inferiorly prolapses out of the brain stem into the ambient cistern and more inferiorly into the cerebellopontine angle abutting and displacing the middle cerebral peduncle.
There is very little surrounding oedema, no calcification or blood product, and no elevation of cerebral blood volume. Spectroscopy demonstrates elevation of choline. The solid enhancing components demonstrate intermediate diffusion restriction (1400 x 10^-6 mm^2/s).
The remainder of the brain is unremarkable, there is no hydrocephalus. No other lesions identified.
Conclusion: Mixed solid cystic tumour arising from the left side of the midbrain and extending into the ambient cistern and cerebellopontine angle most likely represents a pilocytic astrocytoma.