Neuroendocrine tumours (NETs) are slow growing and may secrete bioactive peptides and hormones. They can arise anywhere in the body, although most commonly in the gastrointestinal tract, lungs, pancreas, pituitary (see Figure 48.1), parathyroid and thyroid. They are usually benign with symptoms resulting from hormone production, but malignant transformation can result.
NETs may be generally categorised by site of origin, histological appearance, embryological origin, or grade of the tumour. There is dispute as to a single classification system, but currently site of origin is used, with detail given as to the humoral capabilities of the tumour.
Functioning NETs produce bioactive substances that produce clinical symptoms, but if originating within the gastrointestinal tract they require the presence of liver metastasis to produce symptoms due to the effect of first-pass metabolism. Non-functioning NETs may secrete bioactive substances but often at subclinical levels and therefore are not associated with a particular hormonal syndrome. The most common symptoms are due to the physical effects of the tumour and non-specific symptoms may be present.
Argentaffin and hormone secretion
NETs from a particular anatomical origin often show similar behaviour as a group, such as tumours arising in the foregut (includes oesophagus, stomach, duodenum, liver, pancreas, thymus, lung), midgut (bowel from jejunum to transverse colon, appendix) and hindgut (descending colon, sigmoid, rectum).
- Foregut NETs are argentaffin negative. Despite low serotonin content, they often secrete 5-hydroxytryptophan (5-HTP), histamine and several polypeptide hormones. There may be associated atypical carcinoid syndrome, acromegaly, Cushing’s disease, other endocrine disorders, telangiectasia, or hypertrophy of the skin in the face and upper neck. These tumours can metastasise to bone.
- Midgut NETs are argentaffin positive, producing high levels of serotonin (5-HT), kinins, prostaglandins, substance P and other vasoactive peptides, and occasionally ACTH. Bone metastasis is uncommon.
- Hindgut NETs are argentaffin negative and rarely secrete 5-HT, 5-HTP, or any other vasoactive peptides. Bone metastases are common.
Carcinoid tumours
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