Neuroendocrine neoplasms of the gastrointestinal tract: classification, pathologic basis, and imaging features.

Many radiologists are unfamiliar with the recently developed World Health Organization classification scheme for neuroendocrine tumors. According to this classification scheme, neuroendocrine tumors are divided into well-differentiated endocrine tumors (carcinoids), well-differentiated endocrine carcinomas (malignant carcinoids), and poorly differentiated endocrine carcinomas on the basis of their location, histologic features, and biologic behavior. Most neuroendocrine tumors have nonspecific imaging characteristics. However, they sometimes have peculiar clinical manifestations and radiologic features, on the basis of which radiologists may infer the specific diagnosis. Neuroendocrine tumors of the gastrointestinal tract originate from the cells derived from the embryonic neural crest, neuroectoderm, and endoderm. They usually produce bioactive substances and show immunoreactivity to neuroendocrine markers. Although neuroendocrine tumors are uncommon, they should be considered in developing the differential diagnosis for gastrointestinal tumors in patients with a typical syndrome or when the tumors have characteristic imaging features.