Tumors of the neuroendocrine system represent a separate group of tumors with a characteristic histological structure and functional properties occurring at various sites in the body, particularly in the gastrointestinal sys­tem and in the lungs. Although they are usually primary slowly growing tumors with the indolent behavior, all of them have malignant potential, which in many cases is difficult to predict and they can metastasise despite the small size of the primary tumor. At present, there is no uniform, universal classification and neuroendocrine ne­oplasms terminology. It differs depending on the location of the tumor and is based primarily on two WHO clas­sifications, the classification of the neuroendocrine neoplasms from the gastropancreatic system from 2010 and the lungs and thymus from 2015. The prognosis depends on the primary tumor localization and its size, as well as on the histological properties, particularly on the proliferation activity characterized by the presence of mitoses and Ki67 expression.

Keywods: neuroendocrine neoplasms, classification, histological characteristics of tumors