Answer: Pituicytoma

Histology: The lesion is composed of plump spindle shaped cells with eosinophilic cytoplasm growing in a storiform pattern. Mitotic figures and necrosis are not evident. The lesion has uniform cellularity throughout. Stain for S-100 was positive, while no intercellular type 4 collagen was identified. Stain for neurofilament was negative, and GFAP stain was focally positive.

Discussion: A schwannoma would share the spindle cell morphology and S-100 positivity with the current lesion. However, a schwannoma typically shows pericellular type 4 collagen deposition, features variable cellularity (Antoni A and Antoni B areas), and Verocay bodies. A meningioma could have a spindled pattern like the current lesion; however, meningiomas are typically diffusely EMA positive in a membranous pattern, and GFAP negative. The normal posterior pituitary features the same spindled pituicytes as the current lesion. However, normal posterior pituitary (neurohypophysis) contains axons, which label with neurofilament, within perivascular anucleate zones, and Herring bodies, which are axonal dilatations.

Pituicytoma is a rare circumscribed tumor of the specialized glial cells of the stalk and posterior lobe of the pituitary gland, known as pituicytes. The tumor pushes aside native neuronal processes in this region, and hence is neurofilament negative. The lesion has a benign course when completely excised.