20 - Histologic Features of Pituitary Adenomas and Sellar Region Masses

Abstract

This chapter focuses on surgical pathologic features of pituitary adenomas using the new classification systems that employ transcription factors steroidogenic factor-1 (SF1) and PIT1 in conjunction with immunostaining for specific anterior pituitary hormones and cytokeratin. Normal pituitary gland histology, general features of pituitary adenomas, molecular genetics of adenomas, and the issue of “atypical adenoma,” which has now been removed from the newest World Health Organization (WHO) classification system (2017), are discussed. Practical issues such as the fact that clinically silent adenomas are not identical to hormone-negative adenomas and why subtyping is valuable in predicting response to therapies are addressed. The histologic and immunohistochemical features expected in specific pituitary adenoma subtypes are presented succinctly and fully illustrated, including the newly added category of plurihormonal PIT1-positive adenoma. Finally, rarer sellar region entities are also included and illustrated, including hypophysitis, Rathke cleft cyst, and craniopharyngiomas (the two types now known to be driven by separate genetic events: beta catenin [CTNNB1] mutation in adamantinomatous and BRAF V600E in papillary). Pituicytoma, spindle cell oncocytoma, and granular cell tumor of the neurohypophysis, WHO grade I sellar region masses now known to be linked by their mutual shared immunoreactivity to thyroid transcription factor-1 (TTF-1), and rare sellar region atypical teratoid/rhabdoid tumor (AT/RT), an entity with a peculiar tendency to affect older adult women, are all pictorially represented. This chapter should facilitate workup of the common pituitary adenoma, as well as rare sellar region masses.