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.
Keywords: Aggressive adenomas; Nonfunctioning adenomas; Pituitary adenomas; Transcription factor; WHO classification.