Neuroendocrine tumor

Neuroendocrine tumors (NETs) are neoplasms that arise from cells of the endocrine (hormonal) and nervous systems. They most commonly occur in the intestine, where they are often called carcinoid tumors, but they are also found in the pancreas, lung, and the rest of the body. Although there are many kinds of NETs, they are treated as a group of tissue because the cells of these neoplasms share common features, including a similar histological appearance, having special secretory granules, and often producing biogenic amines and polypeptide hormones. The term "neuro" refers to the dense core granules (DCGs), similar to the DCGs in the serotonergic neurons storing monoamines. The term "endocrine" refers to the synthesis and secretion of these monoamines. The neuroendocrine system includes endocrine glands such as the pituitary, the parathyroids and the neuroendocrine adrenals, as well as endocrine islet tissue embedded within glandular tissue such as in the pancreas, and scattered cells in the exocrine parenchyma. The latter is known as the diffuse endocrine system. The World Health Organization (WHO) classification scheme places neuroendocrine tumors into three main categories, which emphasize the tumor grade rather than the anatomical origin: well-differentiated neuroendocrine tumors, further subdivided into tumors with benign and those with uncertain behavior well-differentiated (low grade) neuroendocrine carcinomas with low-grade malignant behavior poorly differentiated (high grade) neuroendocrine carcinomas, which are the large cell neuroendocrine and small cell carcinomas. Additionally, the WHO scheme recognizes mixed tumors with both neuroendocrine and epithelial carcinoma features, such as goblet cell cancer, a rare gastrointestinal tract tumor. Placing a given tumor into one of these categories depends on well-defined histological features: size, lymphovascular invasion, mitotic count, Ki-67 labelling index, invasion of adjacent organs, presence of metastases and whether they produce hormones.