Introduction to neuroendocrine neoplasms of the digestive system: definition and classification

Neoplasms characterized by the expression of markers of neuroendocrine differentiation in neoplastic cells are defined neuroendocrine. This broad definition comprises tumors found at different sites of the body with similar morphology but different behavior and genetic background. From a clinical standpoint neuroendocrine neoplasms may be functioning, when they give rise to unregulated secretion of hormones. Functioning tumors account for about one-third of neuroendocrine neoplasms. From a pathological standpoint neuroendocrine neoplasm are classified by cancer category, cancer families/classes, cancer types, cancer grade and cancer stage. The category identifies the cancer major trait and thus defined as neuroendocrine neoplasia (NEN) to comprise all families/classes of neuroendocrine cancer. The cancer family/types are neuroendocrine tumors (NET) as well differentiated, and neuroendocrine carcinoma (NEC) as poorly differentiated forms. Cancer grade, based on proliferation measure by mitotic count and Ki-67%, and cancer stage, based on tumor size and invasion (T), node deposits (N) and distant metastases (M), complete the pathological classification. Site-specific differences are the rule. Still missing is a genetic classification tool to complement current pathological descriptors.