The World Health Organization classifications of pituitary neuroendocrine tumours: a clinico-pathological appraisal

The classification of tumours of the pituitary gland has recently been revised in the 2021 5th edition World Health Organization (WHO) Classification of Central Nervous System Tumours (CNS5) and 2022 5th edition WHO Classification of Endocrine and Neuroendocrine Tumours (ENDO5). This brief review aims to appraise the most relevant changes and updates introduced in the two classifications. A new nomenclature has been introduced in CNS5 and ENDO5 to align adenohypophyseal tumours with the classification framework of neuroendocrine neoplasia. The term pituitary neuroendocrine tumour (PitNET) with subtype information has therefore been adopted and preferred to adenoma. Pituitary carcinoma has been replaced by metastatic PitNET. The ICD-O coding has been changed from benign to malignant in line with NETs from other organs. Histological typing and subtyping based on immunohistochemistry for lineage-restricted pituitary transcription factors are regarded as the cornerstone for accurate classification. Such an approach does not fully reflect the complexity and dynamics of pituitary tumorigenesis and the variability of transcription factors expression. ENDO5 does not support a grading and/or staging system and argues that histological typing and subtyping are more robust than proliferation rate and invasiveness to stratify tumours with low or high risk of recurrence. However, the prognostic and predictive relevance of histotype is not fully validated. Recent studies suggest the existence of clinically relevant molecular subgroups and emphasize the need for a standardized, histo-molecular integrated approach to the diagnosis of PitNETs to further our understanding of their biology and overcome the unsolved issue of grading and/or staging system.