Neuroendocrine tumours and carcinoid syndrome: the AU general practice approach

Neuroendocrine tumours (NETs) — epithelial neoplasms with neuroendocrine differentiation — are more common than most GPs appreciate. Australian prevalence is approximately 70 per 100,000, exceeding that of gastric, pancreatic, and oesophageal cancers combined, with incidence rising 5–7% per year largely due to improved detection. They arise most commonly from the small bowel (midgut, ~30%), pancreas (~25%), lung (~25%), rectum, appendix, and stomach. NeuroEndocrine Cancer Australia (NECA) and Therapeutic Guidelines (eTG) both note that the average diagnostic delay is 5–7 years from first symptom — a gap that general practice is uniquely positioned to close.

The clinical spectrum is wide. Well-differentiated G1/G2 tumours are often indolent; median survival for metastatic G1/G2 NETs on modern therapy is 5–10 years. Poorly differentiated neuroendocrine carcinoma (NEC) is aggressive and treated like small-cell lung cancer. Between these extremes sits a large group of patients who carry a NET diagnosis for years, manage their condition with somatostatin analogues, and rely on their GP for coordination, monitoring, and anticipatory care.

A. Core clinical — the AU general-practice framework

Classification and grading

The WHO 2019/2022 classification grades neuroendocrine neoplasms by Ki-67 proliferation index and mitotic count. NET G1 (Ki-67 <3%, mitotic count <2 per 10 HPF) is the most common and most indolent form. NET G2 (Ki-67 3–20%) has intermediate behaviour. NET G3 (Ki-67 >20%) is well-differentiated but aggressive; somatostatin analogues and PRRT may still apply. Poorly differentiated neuroendocrine carcinoma (NEC) — small-cell or large-cell — has Ki-67 typically >55% and is managed as a distinct entity with platinum-based chemotherapy.

Legacy anatomical terms describe origin: foregut (lung, thymus, stomach, duodenum, pancreas), midgut (jejunum, ileum, appendix, proximal colon), and hindgut (distal colon, rectum). Midgut NETs most commonly cause carcinoid syndrome when metastatic. Rectal NETs, the most frequently incidentally detected NETs at colonoscopy, very rarely produce functional syndromes.

About 5–10% of NETs are hereditary: MEN1 (MEN1 gene — parathyroid + pituitary + pancreatic NET), VHL (VHL — pancreatic NET + renal cell carcinoma), NF1 (NF1 — duodenal somatostatinoma + phaeochromocytoma), and tuberous sclerosis. A family history of any of these syndromes should prompt earlier and more thorough investigation of NET-suggestive symptoms. See the ENETS 2024 guideline on small intestine NET for the full classification framework.

Carcinoid syndrome — recognition in general practice

Carcinoid syndrome results from systemic serotonin and tachykinin release by a NET. It almost always requires liver metastasis from a midgut primary — hepatic monoamine oxidase clears portal-vein serotonin from the gut, so systemic effects only emerge when tumour deposits in the liver drain directly into the systemic venous circulation. The four classic features are:

1. Episodic flushing — facial and upper-chest erythema; dry (distinguishing it from menopausal flushes, which are diaphoretic); provoked by alcohol, emotional stress, exertion, spicy food, or physical pressure over the tumour.
2. Secretory diarrhoea — watery, voluminous, often 5+ episodes per day; distinguished from IBS by its nocturnal occurrence, the absence of bloating, and weight loss.
3. Wheeze / bronchospasm — in approximately 15% of cases; can mislead a clinician toward asthma.
4. Carcinoid heart disease (Hedinger syndrome) — fibrous plaque deposition on right-sided heart valves produces tricuspid regurgitation and pulmonary regurgitation. Present in 20–50% of patients with prolonged carcinoid syndrome exposure. New right-sided murmur in a patient with recurrent diarrhoea and flushing is a diagnostic red flag.

Other functional NET syndromes arise mainly from pancreatic primaries: insulinoma (Whipple’s triad — fasting hypoglycaemia relieved by glucose), gastrinoma / Zollinger-Ellison syndrome (refractory peptic ulcer disease with diarrhoea, MEN1 association), VIPoma (large-volume secretory diarrhoea with hypokalaemia), and glucagonoma (necrolytic migratory erythema + weight loss).

Red flags for investigation

Consider NET investigation in any patient with:

• Recurrent unexplained flushing, watery diarrhoea, wheeze, or right-sided cardiac disease without clear alternative cause.
• Irritable bowel syndrome with nocturnal diarrhoea, unintentional weight loss, or abnormal inflammatory markers.
• A lung nodule identified incidentally on CT — bronchopulmonary carcinoids account for ~25% of NETs.
• A hypervascular liver lesion of unknown primary on imaging.
• First-degree relative with MEN1, VHL, NF1, or tuberous sclerosis.

B. Investigations — the diagnostic pathway

Biochemical markers

24-hour urine 5-hydroxyindoleacetic acid (5-HIAA) is the primary screen for carcinoid syndrome, with sensitivity 70–90% and specificity ~90%. The patient restricts serotonin-rich foods for 48 hours before and during the collection (bananas, walnuts, pineapple, avocado, tomatoes, kiwifruit, plums, aubergine) and avoids paracetamol, salicylates, levodopa, MAOIs, and NSAIDs which can falsely elevate or suppress results. Two separate collections increase diagnostic yield.

Plasma chromogranin A (MBS item 66829) has sensitivity around 80% and specificity 95% but is most useful for serial monitoring rather than initial diagnosis. Critically, proton pump inhibitor use markedly elevates chromogranin A — a value of two to ten times the upper limit of normal from PPI use can closely mimic a true positive. Chromogranin A for diagnostic purposes requires PPI washout of at least two weeks.

For specific functional syndromes: a fasting gut hormone panel (gastrin off PPI, insulin and C-peptide during supervised 72-hour fast for insulinoma), VIP, glucagon, and somatostatin are ordered in the appropriate clinical context. Niacin (vitamin B3) deficiency — from tryptophan diversion to serotonin production — can occur in carcinoid syndrome and present as pellagra (dermatitis, diarrhoea, dementia).

Imaging

CT chest/abdomen/pelvis with multiphase contrast is the primary staging investigation. MRI liver with hepatocyte-specific contrast is the most sensitive modality for hepatic metastases. Ga-68 DOTATATE PET/CT (MBS item 61632) is the gold standard for somatostatin receptor (SSTR)-expressing G1/G2 NETs — it identifies the primary if unknown, detects occult metastases, and confirms SSTR expression needed for prescribing somatostatin analogues and PRRT. Per ENETS 2024 and NCCN guidelines, Ga-68 DOTATATE PET/CT has replaced octreotide scintigraphy as the functional imaging standard. ¹⁸F-FDG PET is used for G3 and NEC where SSTR expression is typically lost and FDG avidity reflects aggressive metabolic behaviour.

Tissue diagnosis

Biopsy with haematoxylin-eosin staining plus immunohistochemistry (chromogranin A, synaptophysin, CD56, INSM1) plus Ki-67 index and mitotic count is required to grade the tumour and guide therapy. Clinical genetics referral is appropriate for pancreatic NET diagnosed under 40 years, multifocal primaries, or features of hereditary syndrome.

C. Treatment overview

Somatostatin analogues — first-line for functional and non-functional G1/G2 NETs

Two pivotal trials established somatostatin analogues as standard care. PROMID (Rinke et al., JCO 2009) demonstrated time-to-progression benefit for octreotide LAR in midgut NETs. CLARINET (Caplin et al., NEJM 2014) showed lanreotide significantly prolonged progression-free survival in G1/G2 GEP NETs compared with placebo, independent of functional status. Both octreotide LAR 30 mg IM monthly (Sandostatin LAR) and lanreotide 120 mg deep subcutaneous monthly (Somatuline Autogel) are PBS-listed for eligible patients with confirmed NET. Breakthrough carcinoid symptoms are managed with short-acting subcutaneous octreotide.

For refractory diarrhoea despite optimal SSA dose, telotristat ethyl (tryptophan hydroxylase inhibitor) is TGA-registered but not PBS-listed, with significant cost implications (approximately $5,000 per month privately).

PRRT — second-line for progressive SSTR+ disease

NETTER-1 (Strosberg et al., NEJM 2017) established lutetium-177 DOTATATE (Lutathera/¹⁷⁷Lu-DOTATATE) as a highly effective second-line therapy for progressive midgut NETs: 65% progression-free at 20 months versus 11% for high-dose octreotide. PRRT is available at major Australian tertiary centres (Royal Brisbane, Royal Melbourne, Peter MacCallum, Royal Adelaide, Royal Prince Alfred, Royal North Shore) through TGA Special Access Scheme; MSAC was evaluating PBS listing as of mid-2026. PRRT requires confirmed SSTR expression on Ga-68 DOTATATE PET/CT.

Carcinoid crisis prevention

Carcinoid crisis — life-threatening haemodynamic collapse with profound flushing, bronchospasm, and arrhythmia — can be precipitated by any procedure that stimulates the tumour or increases circulating serotonin. Prevention requires continuous intravenous octreotide (50–100 micrograms per hour) initiated one hour before and continued for several hours after any surgery, biopsy, embolisation, or interventional radiology procedure. GPs must flag this risk explicitly on any procedural referral for a patient with known carcinoid syndrome.

D. Australian operations

Referral pathway

All confirmed NETs should be referred to a multidisciplinary NET clinic, available at major tertiary centres. ENETS Centres of Excellence demonstrates that outcomes — including overall survival — are significantly better at high-volume specialised centres. In Australia, NeuroEndocrine Cancer Australia (NECA) maintains a directory of treating centres and provides patient navigation support.

The GP’s role in shared care includes: monitoring chromogranin A and urine 5-HIAA serially, identifying side effects of somatostatin analogues (injection-site reactions, cholelithiasis, glucose intolerance), screening for carcinoid heart disease with annual echocardiography and NT-proBNP if carcinoid syndrome is present, managing nutritional deficiencies (niacin, fat-soluble vitamins from secretory diarrhoea), and coordinating annual echocardiographic surveillance.

MBS items relevant to GP management

Standard consultations MBS items 23 / 36 / 44; chromogranin A 66829; Ga-68 DOTATATE PET/CT 61632; CT with contrast; GPCCMP (items 965/967) for chronic disease management plan.

PBS listings

Octreotide LAR and lanreotide are PBS-listed for eligible patients with confirmed NET under Section 85. Everolimus (RADIANT-3 / RADIANT-4) is PBS-listed for progressive pancreatic or extra-pancreatic non-functional NETs after SSA progression.

E. Special populations

MEN1. Pancreatic NETs in MEN1 are often multifocal and can be managed watchfully when small (<2 cm, non-functional) with active surveillance. Annual MEN1 screening — calcium, PTH, fasting gut hormones, MRI pancreas — is managed through endocrinology. Screen first-degree relatives.

Appendiceal NETs. Most are incidental findings at appendicectomy. Tumours ≤2 cm are almost always cured by appendicectomy alone. Tumours >2 cm require right hemicolectomy and staging. GPs should ensure operative histology is reviewed by a NET multidisciplinary team.

Bronchopulmonary carcinoid. Typical carcinoids (Ki-67 <2%) have excellent prognosis after resection. Atypical carcinoids behave more aggressively. Ectopic ACTH production can cause Cushing’s syndrome — see the eTG Endocrine guidelines for the ectopic ACTH workup.

Carcinoid heart disease. Right-sided valvular surgery improves survival in selected patients with severe carcinoid heart disease at experienced cardiac surgical centres. Cardiology co-management and serial echocardiography are standard.

When to escalate

Refer urgently to emergency care for suspected carcinoid crisis — haemodynamic instability with acute flushing and bronchospasm in a patient with known or suspected NET. Refer as a new patient to a multidisciplinary NET clinic for any confirmed or strongly suspected NET. Refer to endocrinology or cardiology for new right-sided murmur in a patient with known carcinoid syndrome. Refer to clinical genetics for suspected hereditary NET syndrome. Escalate to oncology when disease progression on somatostatin analogue is suspected based on rising 5-HIAA, chromogranin A, or new imaging findings.

What this article is and is not

This is general health information drawn from current guidelines — including ENETS 2024, NCCN Neuroendocrine Tumour Guidelines, Therapeutic Guidelines (eTG), and NeuroEndocrine Cancer Australia — and pivotal clinical trials (NETTER-1, CLARINET, PROMID). It is not personal medical advice and does not create a doctor–patient relationship. Decisions about investigation, treatment choice, and specialist referral are made with the treating GP, endocrinologist, and oncology team.

For Australian consumer-facing information: NeuroEndocrine Cancer Australia (NECA), HealthDirect — neuroendocrine tumours, Better Health Channel — carcinoid tumours.

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Sources cited

1. ENETS 2024 — well-differentiated small intestine NET
2. ENETS 2022 — carcinoid syndrome and carcinoid heart disease guidelines

3. NCCN — Neuroendocrine and Adrenal Tumors Guidelines
4. Strosberg et al. NETTER-1 — ¹⁷⁷Lu-DOTATATE in midgut NET (NEJM 2017)
5. Caplin et al. CLARINET — lanreotide in GEP NET (NEJM 2014)
6. Rinke et al. PROMID — octreotide LAR in midgut NET (JCO 2009)
7. NeuroEndocrine Cancer Australia (NECA)
8. Therapeutic Guidelines (eTG) — Endocrine: neuroendocrine tumours
9. HealthDirect — neuroendocrine tumours
10. Better Health Channel — carcinoid tumours