A large cystic lesion arises from the pituitary fossa, which it expands and remodels, and compresses the optic chiasm. It is fluid-filled but does not suppress on FLAIR. The anterior pituitary tissue is stretched over the cyst.
Features are most consistent with a Rathke's cleft cyst. A cystic macroadenoma and a craniopharyngioma are much less likely.