Changes in the concept of neuroendocrine tumor

Neuroendocrine tumor is defined as the tumor associated with neuroendocrine differentiation. There have been confusions regarding the concept of neuroendocrine tumor. This has been especially complicated by the long-standing concept of "Karzinoide Tumor" proposed by Oberndofer. Neuroendocrine differentiation has been examined by histochemical methods such as Grimelius method or identification of neurosecretory granules but is in general characterized by immunohistochemistry of neuroendocrine markers such as chromogranine A. In addition, neuroendocrine tumor is currently classified into: 1. Well-differentiated endocrine tumor; 2. Well-differentiated endocrine carcinoma; 3. Poorly-differentiated endocrine (small cell) carcinoma; 4. Mixed exocrine-endocrine tumor; and 5. Tumor-like lesion according to its degree of differentiation, cell proliferation or other histological features. Especially, cell proliferation determined by Ki67/MIB1 labeling index plays important roles in this classification. This new classification is important not only in predicting the prognosis of patients with neuroendocrine tumor but also in defining the modes of treatment, and it has become important for those involved in clinical management of these patients to be familiar with this classification.