12. Advances in the diagnosis and treatment of gastrointestinal neuroendocrine tumors

Gastrointestinal neuroendocrine tumors are rare neoplasms that share a common histology and biochemical features yet differ widely in their natural histories. Recent advances in our understanding of the biology of these tumors have been helpful in designing diagnostic localization strategies, selecting patients for appropriate treatment, and developing novel therapeutic approaches. Gastrointestinal neuroendocrine tumors constitute approximately 2% of all malignant gastrointestinal tumors [1]. They are classically divided into the carcinoid tumors and the pancreatic islet cell tumors. Neuroendocrine tumors have been described as being comprised of APUD (amine-precursor uptake and decarboxylation) cells [2,3]. A fundamental unifying concept is that neuroendocrine tumor cells have vesicles containing peptides that can act in an endocrine or paracrine fashion [4]. It is important to note, however, that not all neuroendocrine tumors are of neural crest origin [5]. Furthermore, a universal neuroendocrine marker has not been identified [6]. In addition to gastrointestinal neuroendocrine tumors, the neuroendocrine tumor family can be most broadly described to also include multiple endocrine neoplasia (MEN) type I-associated pituitary adenomas, MEN II-associated medullary thyroid carcinoma and pheochromocytomas, as well as paragangliomas, parathyroid adenomas, melanomas, Merkel cell tumors, and small cell lung carcinomas [4]. While neuroendocrine tumors, including those of gastrointestinal origin, are diverse in terms of their natural histories, there are some common aspects of the tumor biology of carcinoids and pancreatic islet cell tumors that have influenced recent developments in diagnostic localization and therapy.