Well-differentiated pancreatic neuroendocrine tumours (PanNETs) and poorly differentiated pancreatic neuroendocrine carcinomas (PanNECs): concepts, issues and a practical diagnostic approach to high-grade (G3) cases.

With increasing accessibility and advancements in abdominal imaging modalities, the incidence of pancreatic neuroendocrine neoplasms has increased steadily during the past few decades. By definition, neuroendocrine neoplasms of the pancreas show neuroendocrine differentiation, but they represent a broad and heterogeneous group of neoplasms with diverse clinical and pathological characteristics. The majority of pancreatic neuroendocrine neoplasms can be classified as well-differentiated pancreatic neuroendocrine tumours (PanNETs) or poorly differentiated pancreatic neuroendocrine carcinomas (PanNECs). While PanNETs and PanNECs are distinct entities with respect to clinical presentation, outcome and therapeutic approach, they may exhibit overlapping histopathological features. Moreover, the frequent modifications in nomenclature and prognostic grading systems over the years of not only pancreatic neuroendocrine neoplasms, but neuroendocrine neoplasms from other organ sites, has created confusion for both pathologists and clinicians as to the appropriate use of terminology and grading when evaluating these neoplasms. This review examines the current concepts and issues of nomenclature and grading of PanNETs and PanNECs. In addition, considering the morphological overlap between high-grade (G3) PanNETs and PanNECs, we discuss an integrative and practical diagnostic approach to aid in discriminating challenging cases.