44 - Pituitary Gland

Abstract

The overwhelming majority of sellar region masses are pituitary adenomas (85%), followed by craniopharyngioma (3%), Rathke cleft cyst (2%), meningioma (1%), and metastases (0.5%); other lesions such as hypophysitis, pituicytoma, spindle cell oncocytoma, and granular cell tumor of neurohypophysis are rare. However, when these other sellar masses do occur, most so closely mimic pituitary adenoma on neuroimaging studies that the clinical/neuroimaging diagnosis is incorrect, and thus diagnosis falls to the surgical pathologist, necessitating discussion of their histologic features. Today, immunohistochemistry (IHC) has largely supplanted electron microscopy (EM) for much of general surgical pathology, and while there is still a possible role for the latter in diagnosing very rare subtypes of pituitary adenomas, the chapter is directed toward practicing surgical pathologists who generally do not have ready access to EM. Antibodies specific for anterior pituitary hormone immunohistochemical antibodies usually allow ready subclassification of pituitary adenomas, but subtyping is significantly augmented by the use of IHCs directed against transcription factors (PIT1 [pituitary transcription factor-1] for prolactin, growth hormone, and/or thyroid-stimulating hormone [TSH] immunoreactive tumors, SF1 [steroidogenic factor-1] for gonadotroph adenomas, TPIT for corticotroph adenomas). The terminology of “null cell” adenoma today is restricted to uncommon hormone-negative/transcription-factor negative adenomas. Thyroid transcription factor 1 (TTF1) shows nuclear immunoreactivity in the vast majority of pituicytomas, spindle cell oncocytomas, and granular cell tumors of neurohypophysis and further aids in diagnosis.