New WHO classification of pituitary adenomas (4th edition): assessment of pituitary transcription factors and the prognostic histological factors

WHO classification of pituitary adenomas was revised in 2017. The two major and significant changes are discussed. (1) The new classification focuses on adenohypophysial-cell lineage for the designation of adenomas, and thus, assessment of pituitary transcription factors is recommended. Its appropriate use has a complementary role in obtaining an accurate diagnosis, particularly in hormone-negative adenomas. Subclassification of nonfunctioning adenomas was revised accordingly and, consequently, null cell adenomas became quite rare. (2) Atypical adenoma, a previous category, was eliminated due to the poor reproducibility and predictive value. Assessment of tumor proliferation marker and other clinical parameters such as invasion are recommended to predict aggressiveness. High-risk adenomas are those with rapid growth, radiological invasion, and a high Ki-67 proliferation index, whereas some special adenoma subtypes commonly show aggressive behavior.