1. Classification of Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) are made by transformed cells producing hormone/mediators and characterized by the shared expression of some neural-specific antigens. NENs may arise in nerve structures, endocrine organs, and in the diffuse neuroendocrine system at various organs and apparata. The rarity, complexity, and diversity of NENs impaired the development of shared definition and classification. Organ-specific classifications have been historically developed and are in use and formalized under the World Health Organization authority. Common classification themes however do exist including common morphology features for most well-differentiated low-grade tumors and for high-grade, poorly differentiated carcinomas, though with diverse prevalence at different anatomical sites. Common features and specific classifications schemes are briefly discussed here.