Newer Concepts in the Classification of Pituitary Adenomas

The classification of tumors of the pituitary gland has seen several changes in recent years. The 4^th edition of the World Health Organization Classification of Tumours of Endocrine Organs, published in 2017, saw the introduction of a classification based on cell lineage using immunohistochemistry for both pituitary specific hormones as well as pituitary specific transcription factors. The term "hormone-producing" was thus replaced by "-troph." The other major change was that the entity of "atypical adenoma," which was introduced in the 2004 classification, to identify tumors with a poor prognosis, was removed as it failed to identify aggressive tumors. Instead, assessment of markers of proliferation, clinical parameters, such as invasive status and histological subtypes were recommended to identify tumors with aggressive potential. The diagnostic criteria for pituitary carcinoma, however, remained unchanged and continued to be defined as a tumor of adenohypophyseal cells that metastasize craniospinally or systemically. Null cell adenomas were more clearly defined as tumors that did not show any cell-type differentiation, lacking immunoexpression of hormones as well as transcription factors. Since 2017, the classification has continued to evolve with the identification of aggressive histological variants. There is more recently a proposal to change the terminology from pituitary adenoma to pituitary neuroendocrine tumor (PitNET). This review summarizes the recent advances in the classification of pituitary adenomas.