Where is this mass centred?
It's centered on the pons - don't be vague and say it's the brainstem.l
What structures is the brainstem composed of?
3 structures make up the brainstem: midbrain, pons and medulla. If you look at this case, carefully, the pons is the epicentre of the tumour.
A huge mass centred on the pons - it expands the brainstem with mass effect on the 4th ventricle, extends into the cerebral peduncles and anteriorly around the basilar artery. The lateral and third ventricles are dilated with prominence of the temporal horns.