The pathologic classification of neuroendocrine tumors: a review of nomenclature, grading, and staging systems

Neuroendocrine tumors (NETs) arise in most organs of the body and share many common pathologic features. However, a variety of different organ-specific systems have been developed for nomenclature, grading, and staging of NETs, causing much confusion. This review examines issues in the pathologic assessment of NETs that are common among primaries of different sites. The various systems of nomenclature are compared along with new proposal for grading and staging NETs. Although differences persist, there are many common themes, such as the distinction of well-differentiated (low and intermediate-grade) from poorly differentiated (high-grade) NETs and the significance of proliferative rate in prognostic assessment. A recently published minimum pathology data set is presented to help standardize the information in pathology reports. Although an ultimate goal of standardizing the pathologic classification of all NETs, irrespective of primary site, remains elusive, an understanding of the common themes among the different current systems will permit easier translation of information relevant to prognosis and treatment.