The evolution in pituitary tumour classification: a clinical perspective

Objective Pituitary tumours comprise a pathologically and clinically diverse group of neoplasms. Classification frameworks have changed dramatically in the past two decades, reflecting improving understanding of tumour biology. This narrative review examines the evolution of pituitary tumour classification, from a clinical perspective. Results In 2004, pituitary tumours were classified as ‘typical’ or ‘atypical’, based on the presence of markers of proliferation, Ki67, mitotic count and p53. In 2017, the new WHO marked a major paradigm shift, with a new focus on lineage-based classification, determined by transcription factor and hormonal immunohistochemistry. The terms ‘typical’ and ‘atypical’ were omitted, though the importance of proliferative markers Ki67 and mitotic count was acknowledged. The recent WHO 2022 classification incorporates further refinements, specifically recognising some less common types that may represent less well-differentiated tumours. Whilst ‘high risk’ tumour types have been identified, further work is still required to improve prognostication. Conclusions Recent WHO classifications have marked significant progress in the diagnostic evaluation of pituitary tumours, though shortcomings and challenges remain for both clinicians and pathologists in managing these tumours.