Primary Pituitary Carcinoids Do Not Exist: A Reappraisal in the Era of Pituitary Neuroendocrine Tumours

The World Health Organization classification of pituitary tumours, published in 2022, supported a change in the terminology from “pituitary adenoma” to “pituitary neuroendocrine tumour” (PitNET). The neuroendocrine cells represent an integral part of the diffuse neuroendocrine system, including, among others, thyroid C cells, the parathyroid chief cells, and the anterior pituitary. Normal and neoplastic adenohypophyseal neuroendocrine cells have light microscopic, ultrastructural features and an immunoprofile compatible with the neuroendocrine cells and neuroendocrine tumours from other organs. Moreover, neuroendocrine cells of pituitary origin express transcription factors which indicate their cell-lineage origin. Thus, pituitary tumours are now considered as a continuum with other neuroendocrine tumours. PitNETs may occasionally be aggressive. In this context, the term “pituitary carcinoid” has no specific meaning: it either represents a PitNET, or a metastasis to the pituitary gland of a neuroendocrine tumour (NET). An accurate pathological evaluation, combined where necessary with functional radionuclide imaging, can define the origin of the tumour. We recommend that clinicians liaise with patient groups to understand the terminology to define primary tumours of adenohypophyseal cells. It is incumbent upon the responsible clinician to explain the use of the word “tumour” in a given clinical context.