Spotlight on Small intestine Neuroendocrine Neoplasms (siNENs)

Disclaimer
This Spotlight is for general education and reassurance only. It cannot replace personalised advice from your own medical team, who understand your individual history, imaging, pathology, and treatment needs. Neuroendocrine tumours vary widely in behaviour, presentation, and management, and guidance may evolve as new evidence emerges. If you have questions about your diagnosis, symptoms, or treatment plan, please discuss them directly with your specialist NET team.

Small intestine neuroendocrine neoplasms (siNENs) – (Jejunum + ileum — excluding duodenum)

Small intestine neuroendocrine neoplasms (siNENs) arise in the mid‑gut, specifically the ileum and jejunum. These two segments form the core of “small intestine NETs” in all major guidelines (ENETS, NANETS, NCCN, ESMO). They share the same biology, behaviour, and clinical course.

Poorly differentiated NECs can occur but are rare; the overwhelming majority are well‑differentiated NETs (G1–G3).

1️⃣ What they are (ileum + jejunum)

Small intestine NETs arise from enterochromaffin cells and are typically slow‑growing but biologically active, often producing serotonin and associated fibrotic complications.

Ileum

The distal (furthest) segment of the small intestine
Average length: ~3.5 metres
“Distal” means closest to the junction with the large intestine, where the ileum meets the cecum at the ileocecal valve
The most common site for small intestine NETs
Most siNETs arise in the terminal ileum, the last 30–40 cm before the ileocecal junction
High density of enterochromaffin cells → serotonin production
Strong association with:
- Multifocality
- Mesenteric fibrosis
- Retroperitoneal fibrosis (less common)
- Carcinoid syndrome

Clarification: These associations reflect serotonin‑driven biology, which is shared by jejunal NETs. The ileum is simply more commonly affected, not biologically different.

Jejunum

The middle segment of the small intestine
Average length: ~2.5 metres
NETs here are less common than in the ileum
When they occur, they behave identically:
- Same multifocality risk
- Same mesenteric fibrosis behaviour
- Same potential for retroperitoneal fibrosis
- Same metastatic patterns
- Same serotonin‑driven biology
- Same treatment pathways

ENETS (2024) and NANETS (2017) classify jejunal and ileal NETs together as mid‑gut NETs because their biology is indistinguishable.

Where does the duodenum fit?

Although the duodenum is anatomically part of the small intestine, it is not included in the “small intestine NET” category.

Different biology
Different grading patterns
Different metastatic behaviour
Different syndromes (e.g. gastrinoma, somatostatinoma)
Different management (often endoscopic)

Duodenal NETs are classified separately as foregut tumours and many are related to gastric/pancreatic issues.

2️⃣ Epidemiology

United Kingdom (England, NCRAS 2018)

Small intestine NETs: 1.46 per 100,000 per year
All NENs (NET + NEC): 8.61 per 100,000 per year

United States (SEER‑based, consensus values)

Small intestine NETs: 1.84 per 100,000 per year
All NETs (all sites): 8.52 per 100,000 per year

3️⃣ Defining features of small intestine NETs

Small intestine NETs have several hallmark features that distinguish them from most other gastrointestinal NENs, particularly in advanced cases.

1. Multifocality

Occurs in 20–40% of patients
Multiple separate primaries along the small bowel
Tumours often very small (<1 cm) and easily missed
Requires careful intraoperative palpation of the entire small bowel

2. Mesenteric fibrosis

A signature feature of mid‑gut NETs. Small intestine NETs frequently trigger a desmoplastic reaction in the mesentery, producing a fibrotic mass around lymph nodes. Mesenteric tumours can be mistaken for primary small intestine neuroendocrine tumours are are usually mesenteric tumour deposits (MTDs), which are dense, fibrous, and large nodal metastases resulting from a small, often undetected, primary tumour mostly found in the ileum. Undetected, they can cause:

Bowel obstruction
Ischaemia
Vascular kinking
Mesenteric retraction
Diagnostic difficulty (primary tumour often invisible on CT)

3. Retroperitoneal fibrosis (RPF)

Less common, but part of the same serotonin‑driven fibrosis spectrum. Often mimics idiopathic RPF but is distinguished by the underlying tumour studies show 8% of all RPF are related to an underlying malignancy (particularly NET and others). The fibrosis can be detected as a mass surrounding the aorta, often at the L4/L5 level. Functional small intestine NETs can cause retroperitoneal fibrosis, which may lead to:

Ureteric obstruction
Hydronephrosis
Renal impairment

RPF is strongly associated with:

Carcinoid syndrome
Elevated 5‑HIAA
Advanced mid‑gut NETs

Key reference: World J Gastrointest Surg. Apr 27, 2023; 15(4): 566-577 Published online Apr 27, 2023. doi: 10.4240/wjgs.v15.i4.566 Kupietzky A, Dover R, Mazeh H. Surgical aspects of small intestinal neuroendocrine tumors. PMID: 37206065 DOI: 10.4240/wjgs.v15.i4.566

4️⃣ Functional behaviour and syndromes

Small intestine NETs produce serotonin and related mediators. Two clinically relevant syndromes are:

Carcinoid syndrome

Mechanism: serotonin and other vasoactive substances bypass hepatic metabolism (usually due to liver metastases).
Features: flushing, diarrhoea, bronchospasm, abdominal pain.
Prevalence: ~20–25% of small intestine NET patients.

Carcinoid heart disease (Hedinger syndrome)

Mechanism: long‑standing serotonin excess causing right‑sided valvular fibrosis.
Features: tricuspid regurgitation, pulmonary valve stenosis, right‑sided heart failure.
Prevalence: 10–20% of those with carcinoid syndrome (≈2–5% of all small intestine NET patients).

5️⃣ Stage at diagnosis

Stage I–II: ~20–25%
Stage III: ~25–30%
Stage IV: ~45–55%

Late presentation is common.

6️⃣ Diagnostic testing (notwithstanding availability)

Biochemical

Chromogranin A (CgA)
24‑hour urinary 5‑HIAA or plasma 5‑HIAA
NT‑proBNP if carcinoid heart disease suspected

Imaging

CT abdomen/pelvis with contrast
CT or MRI liver
68Ga‑DOTATATE PET/CT (somatostatin receptor positive)
Capsule endoscopy or balloon enteroscopy for occult primaries

Pathology

Confirms NET vs NEC and grade (Ki‑67)
CDX2 supports mid‑gut origin
Somatostatin receptor expression

Cardiac

Echocardiogram
NT‑proBNP

7️⃣ Genetics Overview

Germline / Familial Aspects

Familial clustering of small intestine NETs is well‑documented, including a major NIH‑indexed population study showing significantly increased risk in first‑, second‑, and even third‑degree relatives. Hereditary = a known gene you are born with. Familial = it runs in families, but no single gene explains it. For small‑intestine NETs, the evidence strongly supports familial clustering, but not a defined hereditary syndrome. This is exactly what the NIH study demonstrated to date. Emerging research suggests that germline variants in DNA repair genes (e.g., MUTYH) may increase susceptibility in a subset of patients, but penetrance remains uncertain.
No single, high‑penetrance hereditary syndrome (like MEN1 or VHL) has been defined for jejunal/ileal NETs.
Routine germline testing is not universally recommended, but may be considered in younger patients, those with multiple primaries, or those with a strong family history.

Key message for patients

Familial cases exist and research into germline predisposition is ongoing, but for most people with a small intestine NET, the tumour is still considered sporadic.

Somatic (tumour)

CDKN1B mutations (~8%)
Chromosome 18 loss (40–60%)
Overall low mutational burden and few actionable targets

NEC genetics

TP53 and RB1 inactivation
Distinct biology from well‑differentiated NETs

8️⃣ Grade 3 and NEC in the small intestine

NET G3: exceptionally rare, estimated <2% of small intestine NETs.
NEC: ~1–3% of all small intestine NENs.

Most small intestine NENs are G1–G2 NETs; high‑grade disease is an outlier.

9️⃣ Main treatments & options (notwithstanding availability and local guidelines)

Management of small intestine NETs is multimodal and tailored to tumour biology, disease burden, symptoms, and somatostatin receptor expression.

Tumour‑directed therapies

Surgery

Resection of primary tumour(s)
Removal of mesenteric lymph nodes
Relief of obstruction or ischaemia from mesenteric fibrosis
Important even in metastatic disease due to multifocality and mesenteric desmoplasia
Liver resection

Somatostatin analogues (SSAs)

First‑line for tumour control
Reduce serotonin release
Lower 5‑HIAA
Delay disease progression

PRRT (e.g. Lutetium‑177 DOTATATE)

For progressive, SSTR‑positive disease
Improves tumour control and hormonal symptoms

Everolimus

Option for progressive, non‑functional or SSTR‑positive disease

Liver‑directed therapies

Embolisation
Ablation
Histotripsy

NET G3

Treated with NET‑directed therapy (SSA, PRRT, everolimus) – see local guidelines for use of chemotherapy

NEC

Treated with platinum‑based chemotherapy
Biologically distinct from NETs

Carcinoid syndrome treatments

Carcinoid syndrome management focuses on symptom control, hormone suppression, and prevention of complications such as carcinoid heart disease.

1. Somatostatin analogues (SSAs) — normally first‑line

Reduce flushing, diarrhoea, and bronchospasm
Suppress serotonin release
Lower 5‑HIAA
Reduce risk of carcinoid heart disease progression

2. Telotristat ethyl (tryptophan hydroxylase inhibitor)

Reduces serotonin production
Used when diarrhoea persists despite SSA therapy
Particularly helpful in patients with very high 5‑HIAA

3. Liver‑directed therapies

Embolisation, ablation, or resection for liver‑dominant disease
Can significantly reduce hormone output and improve symptoms

4. PRRT (Lutetium‑177 DOTATATE)

Reduces tumour burden
Improves hormonal symptoms
Particularly effective in SSTR‑positive disease

5. Supportive measures

Anti‑diarrhoeal agents
Nutritional support
Monitoring for B3 niacin deficiency (tryptophan diversion) in high risk/active carcinoid syndrome scenarios
Regular echocardiography for carcinoid heart disease surveillance for those at risk

6. Peri‑operative carcinoid crisis prevention

Specialist anaesthetic planning to prevent intraoperative hormone release and haemodynamic instability

🔟 Guideline anchors

ENETS small intestine NET guidance
NANETS mid‑gut NET guidance
NCCN Neuroendocrine & Adrenal Tumors
ESMO GEP‑NEN guidelines

You may also find some of my earlier posts useful:

Ronny Allan’s Spotlight on NENs Series

Disclaimer

I am not a doctor or any form of medical professional, practitioner or counsellor. None of the information on my website, or linked to my website(s), or conveyed by me on any social media or presentation, should be interpreted as medical advice given or advised by me.

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Please also note that mention of a clinical service, trial/study or therapy does not constitute an endorsement of that service, trial/study or therapy by Ronny Allan, the information is provided for education and awareness purposes and/or related to Ronny Allan’s own patient experience. This element of the disclaimer includes any complementary medicine, non-prescription over the counter drugs and supplements such as vitamins and minerals.