What is the main differential which necessitates followup of larger / atypical lesions?
A cystic pineal parenchymal tumour (specifically a pineocytoma) can appear very similar. If a cyst is larger than 10-14 mm many would advocate follow-up to ensure stability over time, although this is contentious.
A cystic pineal gland lesion measuring 8 x 13 x 7 millimetres is demonstrated, which is located above the pineal calcifications, and below the internal cerebral veins. There is no convincing mass effect upon the tectal plate, and no evidence of hydrocephalus. Following the administration of contrast no solid enhancement can be identified.
The remainder of the brain is within normal limits, with no intra or extraaxial collection masses or focal regions of abnormal contrast enhancement noted. Incidental note is made of mucosal thickening in the the paranasal sinuses without evidence of acute sinusitis.
Cystic pineal gland lesion as described most likely represents a benign pineal cyst. Given its size, follow-up is recommended, as a cystic pineocytoma could have similar appearances.