Is the third ventriculostomy patent? How can you tell?
Yes, widely patent. This is best seen on sagittal T2 where a large flow void is seen passing from the third ventricle to the suprasellar / interpeduncular cistern.
What is the most likely aetiology of the pineal region mass?
As it is a well circumscribed mass most likely represents a primary pineal parenchymal neoplasm, most likely a pineocytoma.
A well circumscribed mass is located in the region of the pineal gland. It is a little heterogeneous with multiple small regions of cyst formation (on T2) most marked anteriorly, probably at the site of biopsy. It enhances vividly and compresses the tectum and obstructs the upper part of the aqueduct with resultant hydrocephalus, treated with a third ventriculostomy, widely patent.