Endocrine Tumors of the Gastrointestinal Tract and Pancreas | Harrison's Hematology and Oncology, 3e | AccessBiomedical Science

GENERAL FEATURES OF GASTROINTESTINAL NEUROENDOCRINE TUMORS

Gastrointestinal (GI) neuroendocrine tumors (NETs) are tumors derived from the diffuse neuroendocrine system of the GI tract; that system is composed of amine- and acid-producing cells with different hormonal profiles, depending on the site of origin. The tumors historically are divided into GI-NETs (in the GI tract) (also frequently called carcinoid tumors) and pancreatic neuroendocrine tumors (pNETs), although newer pathologic classifications have proposed that they all be classified as GI-NETs. The term GI-NET has been proposed to replace the term carcinoid; however, the term carcinoid is widely used, and many are not familiar with this change. Accordingly, this chapter will use the term GI-NETs (carcinoids). These tumors originally were classified as APUDomas (for amine precursor uptake and decarboxylation), as were pheochromocytomas, melanomas, and medullary thyroid carcinomas, because they share certain cytochemical features as well as various pathologic, biologic, and molecular features (Table 51-1). It was originally proposed that APUDomas had a similar embryonic origin from neural crest cells, but it is now known the peptide-secreting cells are not of neuroectodermal origin. Nevertheless, the concept of APUDomas is useful because these tumors have important similarities as well as some differences (Table 51-1). In this section, the areas of similarity between pNETs and GI-NETs (carcinoids) will be discussed together, and areas in which there are important differences will be discussed separately.

TABLE 51-1General Characteristics Of Gastrointestinal Neuroendocrine Tumors (GI-NETs [Carcinoids], Pancreatic Neuroendocrine Tumors [pNETs])

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TABLE 51-1 General Characteristics Of Gastrointestinal Neuroendocrine Tumors (GI-NETs [Carcinoids], Pancreatic Neuroendocrine Tumors [pNETs])

Share general neuroendocrine cell markers (identification used for diagnosis)
1. Chromogranins (A, B, C) are acidic monomeric soluble proteins found in the large secretory granules. Chromogranin A is the most widely used.
2. Neuron-specific enolase (NSE) is the γ-γ dimer of the enzyme enolase and is a cytosolic marker of neuroendocrine differentiation.
3. Synaptophysin is an integral membrane glycoprotein of 38,000 molecular weight found in small vesicles of neurons and neuroendocrine tumors.
Pathologic similarities
1. All are APUDomas showing amine precursor uptake and decarboxylation.
2. Ultrastructurally, they have dense-core secretory granules (>80 nm).
3. Histologically, they generally appear similar with few mitoses and uniform nuclei.
4. Frequently synthesize multiple peptides/amines, which can be detected immunocytochemically but may not be secreted.
5. Presence or absence of clinical syndrome or type cannot be predicted by immunocytochemical studies.
6. Histologic classifications (grading, TNM classification) have prognostic significance. Only invasion or metastases establish malignancy.
Similarities of biologic behavior
1. Generally slow growing, but some are aggressive.
2. Most are well-differentiated tumors having low proliferative indices.
3. Secrete biologically active peptides/amines, which can cause clinical symptoms.
4. Generally have high densities of somatostatin receptors, which are used for both localization and treatment.
5. Most (>70%) secrete chromogranin A, which is frequently used as a tumor marker.
Similarities/differences in molecular abnormalities
1. Similarities
  1. Uncommon—mutations in common oncogenes (ras, jun, fos, etc).
    ...