Advances and controversies in the classification and grading of pituitary tumors.

BACKGROUND: Pituitary tumors are common lesions, and they represent the second most frequent primary brain tumor. Their classification has undergone several changes over time. The World Health Organization conducts periodic expert review/consensus meetings and publishes the results as recommendations for changes in classification, based on advances in molecular and genetic advances. This paper summarizes the results of the 2017 WHO Classification, which recommends several important changes.

PURPOSE: This paper provides a review of the major changes and issues leading to an understanding of the basis for a new pituitary tumor classification. They include the rejection and modification of prior conceptual and pathological characteristics of these neoplasms. There is also considerable concern related to invasive and recurrent pituitary tumors which follow a less benign course than the typical pituitary adenoma.

METHODS: A review of the outcome data for the previously designated "atypical" pituitary tumor category revealed that the former criteria were not adequate to support their ability to predict with accuracy the clinical course of a given tumor. A similar review was accomplished regarding the role of the p53 tumor suppressor mutation. Again, there was no reliable contribution of p53 status to tumor aggressiveness. Other changes have occurred regarding the cytogenetic lineage of the various subtypes of pituitary adenoma. The transcription factors Pit-1, SF-1, and TPit play a major role in determining tumor subtypes and have become part of the classification criteria.

RESULTS: These advances now help provide the background for more reliable and consistent classification of pituitary adenomas. Further definition of aggressive characteristics such as cavernous sinus and dural invasion remain to be considered in the quest to make more accurate prognostic projections based on histopathological analysis.

CONCLUSIONS: The 2017 WHO Classification of Pituitary Tumors provides a more solid basis for accurate and reliable prognostic assessment of these lesions. Further progress undoubtedly will be made as the recommendations of this update are incorporated in to routine use.