The 2017 WHO classification of pituitary adenoma: overview and comments.

The fourth edition of the World Health Organization classification of endocrine tumors has been recently published. There are two critical changes to the classification for pituitary adenomas in this edition. One is that the term "atypical adenoma," which was characterized based on highly proliferative properties to predict adenomas that carry a poor prognosis, was completely eliminated due to the lack of definitive evidence. The other change is the introduction of more precise cell lineage-based classification of pituitary adenoma that is defined based on lineage-specific transcription factors and hormones produced. Accordingly, null cell adenomas have been re-defined as those that show completely negative immunostaining either for hormones or for adenohypophyseal transcription factors. In this review, we summarized these changes in the WHO classification and discussed topics that are relevant to the diagnosis of actual cases: immunohistochemical study for pituitary endocrine tumors, predictive markers for malignant potential, the relationship between somatotroph adenomas and somatostatin analogs, and characteristics of plurihormonal adenomas.