GH-Secreting Adenoma or Tumor? Issues in Pituitary Neoplasms Nomenclature, Classification, and Characterization

Acromegaly is a rare disorder characterized by chronic hypersecretion of growth hormone (GH) and, consequently, of its mediator, insulin-like growth factor 1 (IGF-1), due in >95% of the cases to a GH-secreting pituitary adenoma (PA)/Pituitary Neuroendocrine Tumor (PitNET). PAs/PitNETs are extremely heterogeneous for clinical, biochemical, radiological, intra-operative, and histological features and, differently from other histologically benign lesions, can cause significant morbidity because of locally aggressive behavior, resistance/recurrence after treatment, and, although very rarely, metastasization. PAs/PitNETs' classification and nomenclature have consistently changed in the course of time, reflecting knowledges about their complex biology, with the aim of stratifying patient risk and, therefore, uniform therapeutic strategies. According to the last WHO Classification, based on pituitary transcription factors (i.e., Pit-1, TPIT, and SF-1), GH-secreting PAs/PitNETs pertain to the Pit-1-lineage. Several subtypes can be distinguished, i.e., somatotroph (sparsely and densely granulated), mixed (mammosomatotroph, mixed somatotroph-lactotroph, and acidophilic stem cell), and plurihormonal (mature and immature Pit-1 lineage), based on hormone staining at immunohistochemistry and granulation, with distinct clinical and radiological features. Unfortunately, this classification does not fully reflect the spectrum of tumor phenotypes, does not consider the presence of drug-target receptors (i.e., somatostatin), nor molecular features that, on the contrary, have been increasingly demonstrated to influence biological behavior. Therefore, efforts of pituitary expert of the various disciplines are still necessary to reach a more comprehensive and detailed PitNET stratification to improve patient care through precision medicine.