NEUROENDOCRINE TUMOURS
Definition
Historically referred to as carcinoid tumours, neuroendocrine tumours (NETs) arise from neuroendocrine cells, which are widely distributed in the body. They are rare tumours (incidence: 5/100 000) but ‘common’ primary sites include the gastrointestinal tract (especially the appendix and terminal ileum), pancreas and lung.
Aetiology
The
risk factors for tumour development are not well understood. A small proportion
arise on the background of an inherited endocrine cancer syndrome, including
MEN-1 (Chapter 33), von Hippel–Lindau syndrome (associated with pancreatic
NETs), neurofibromatosis type 1 (NF1) and tuberous sclerosis.
Symptoms
and signs
Because
of their wide distribution, the spectrum of presentation is diverse. Many are
discovered incidentally during a diagnostic work-up undertaken forother
reasons. Patients with gastrointestinal NETs can present with bowel obstruction
while fewer than 10% present with classic carcinoid syndrome (characterised by
dry flushing and diarrhoea) (Figure 31.1). This occurs when vasogenic peptides,
including serotonin, gain access to the systemic circulation, most commonly as
a result of liver metastases from an intestinal primary. Patients with
pancreatic NETs present with symptoms related to the tumour mass (abdominal
pain, weight loss) or a hypersecretory syndrome (Figure 31.2).
Diagnosis
Biochemistry
Plasma
chromogranin A is a useful general neuroendocrine marker that has high
sensitivity for most types of NET. Falsely
raised
readings can be seen in patients with renal disease and in those taking proton
pump inhibitor therapy.
Most
intestinal NETs secrete serotonin. Its breakdown product, 5-hydroxyindoleacetic
acid (5-HIAA), can be measured in a 24-hour urine collection and is typically
elevated in patients with intestinal NETs and metastatic liver disease. A
fasting gut hormone profile (measuring gastrin, glucagon, vasoactive intestinal
peptide, somatostatin and pancreatic polypeptide) should be measured in patients
with pancreatic NETs. Patients with suspected insulinoma require a supervised
inpatient fast to establish the diagnosis (Chapter 34).
Radiology
A
variety of imaging modalities are used to help establish the site of the
primary tumour and to document the extent of the disease. Cross-sectional
imaging (CT or MRI) (Figure 31.3) is used commonly while endoscopy, endoscopic
ultrasound and selective venous sampling are useful in selected cases,
especially in patients with pancreatic disease. Somatostatin receptor
scintigraphy (SSRS; Octreoscan®) is
used widely, not only to define the extent of disease, but also to
determine suitability for somatostatin analogue therapy and peptide receptor
radionuclide therapy.
Pathology
This
is considered the gold standard for diagnosis. Immunohistochemistry directed
against a panel of general neuroendocrine markers will confirm a neuroendocrine
origin, supplemented where necessary with immunostaining for specific hormones
(e.g. gastrin in suspected gastrinoma). A key part of the pathology report is
to estimate the proliferative potential and grade of the tumour by measuring
the Ki67 proliferation index and/or mitotic count. This helps determine
prognosis but can treatment choice (e.g. chemotherapy can be used in NETs with a high Ki67 index).
