The sections show fragments of a moderately hypercellular adenoma. Tumour cells have uniform round and oval vesicular and hyperchromatic nuclei and a variable amount of pale cytoplasm which are arranged in trabeculae and diffuse sheets intersected by thin-walled capillaries. No mitotic figures or areas of necrosis are seen and there is no included normal anterior pituitary tissue.

Immunohistochemistry shows strong perinuclear staining for CAM5.2 in the majority of tumour cells. No immunostaining for growth hormone, prolactin, ACTH, FSH, LH, TSH, S-100 protein, GFAP or oestrogen receptor (ER) is seen in tumour cells. The features are of null cell adenoma. The topoisomerase labelling index is <1%.

FINAL DIAGNOSIS: Pituitary tumour:   Null cell adenoma