What is the main differential diagnosis?
Pituitary macroadenoma, papillary craniopharyngioma, lymphocytic hypophysitis, metastasis.
What features are unusual for a macroadenoma?
Probable dural thickening anteriorly, small pituitary fossa (no remodelling), dumbbell shape to the tumour and irregular margins.
What does the absence of fossa enlargement and the dumbbell shape of the tumour imply about its growth?
Both these features suggest rapid growth, without time for bony remodelling or diaphragma sella stretching.
MRI of the pituitary demonstrates a soft tissue mass filling the fossa, extending up along the infundibulum and into the left cavernous sinus.