Neuroendocrine tumor classification systems: Grading

Abstract

Grading and differentiation are the terms used for the pathological classification of tumors. Grading of the tumor is a method to express the biological features and aggressiveness of the tumor. Conversely, the term differentiation is used to reflect the degree of tumor resemblance to its nonmalignant counterpart.WHO has proposed a more scientific and clinically related tumor-based classification in 2000. The WHO classification suggests that neuroendocrine neoplasms are potentially malignant and consequently should be staged according to a site-specific staging system.The World Health Organization (WHO) proposed the first methodical classification of neuroendocrine tumors. Currently, there are a number of other systems available to classify NETs, including NANET (North American NET Society) and ENET (European Neuroendocrine Tumor Society) in addition to the most recent version of WHO classification (2010). These novel systems are notable in that they consider the clinicopathological site-specific differences when classifying a case of NET. Also, unlike previous versions of them, these systems consider a neuroendocrine tumor as a potentially malignant neoplasm.Overall there is a general consensus between these systems with only small differences between NANET and ENET. These differences regard especially the staging systems for pancreatic and appendicular NETs. Most systems use the cell proliferative rate to classify the tumor as low, intermediate, or high grade, In addition to histopathological features, functionality of a neuroendocrine tumor should also be considered. Functional status of a tumor is clinically and may be prognostically important; however, grade and stage are the factors that usually determine the prognosis and the therapeutic approach for a certain NET.In this chapter we review the most common, currently available, classification systems, and, when needed, we discuss the differences between them.