The patient went on to have a resection. 

Histology

The tumor forms nests and vague perivascular pseudorosette-like structures. The tumor cells are bland with oval nuclei, vesicular chromatin, occasional small nucleoli and amphophilic cytoplasm with poorly defined cell outlines. Occasional cells have nuclear grooves or intranuclear pseudo inclusions. There are no mitoses and no necrosis.

IMMUNOHISTOCHEMISTRY:

• The tumor cells are positive for S100 and TTF1.
• GFAP is focal and weak. 
• The tumor cells are negative for ACTH, prolactin, FSH, TSH, LH, growth hormone, PLAP, synaptophysin and AE1/AE3. 
• Ki67 is approximately 2-3%.

FINAL DIAGNOSIS: pituicytoma (WHO grade I).