Gastroenteropancreatic Neuroendocrine Neoplasms (GEPNENs)

What are GEPNENs?

Gastroenteropancreatic neuroendocrine neoplasms (GEPNENs) are a heterogeneous group of tumours that arise from the diffuse neuroendocrine system of the gastrointestinal tract and pancreas. They are characterised by the ability to synthesize, store and secrete peptide hormones and amines, which may cause distinct clinical syndromes.

Epidemiology

Although historically considered rare, the reported incidence of GEPNENs has risen to approximately 57 cases per 100,000 persons per year, largely due to improved imaging and pathological assessment. They can occur at any age, but the median age at diagnosis is 5560years. Slight male predominance is observed in pancreatic NETs, while smallbowel NETs affect both sexes equally.

Classification

The 2019 WHO classification separates GEPNENs into two main categories based on differentiation and proliferative activity:

• Welldifferentiated neuroendocrine tumours (NETs) graded G1, G2, or G3 according to Ki67 index and mitotic count.
• Poorly differentiated neuroendocrine carcinomas (NECs) highgrade (G3) lesions with smallcell or largecell morphology.

Grading criteria:

Common Primary Sites

• Small intestine (especially the ileum) most frequent site.
• Pancreas second most common.
• Appendix, stomach, colon, rectum and rarely the biliary tree.

Clinical Presentation

Symptoms fall into two groups:

1. Hormonerelated (functional) syndromes caused by excess secretion of active peptides.
2. Massrelated (nonfunctional) symptoms due to tumour bulk or metastasis.

Functional Syndromes

Nonfunctional Presentations

Many GEPNENs are discovered incidentally during imaging for unrelated reasons. When symptomatic, patients may have:

• Abdominal pain or mass effect.
• Obstruction or intussusception (particularly in the small bowel).
• Jaundice (pancreatic head lesions).
• Unexplained weight loss.
• Metastatic disease most commonly to the liver, then to lymph nodes and bone.

Diagnostic Workup

1. Biochemical Testing
   • Chromogranin A (CgA) a sensitive marker for most NETs.
   • 5Hydroxyindoleacetic acid (5HIAA) in 24hour urine for serotoninproducing tumours.
   • Specific hormone assays when a functional syndrome is suspected (e.g., gastrin, insulin, VIP).
2. Imaging
   • Multiphasic contrastenhanced CT or MRI firstline for localisation and staging.
   • 68GaDOTATATE PET/CT gold standard for somatostatinreceptor imaging; detects lesions as small as 5mm.
   • 68GaDOTATOC/DOTANOC PET may be used when DOTATATE is unavailable.
   • In selected cases, 18FFDG PET is useful for highgrade NECs.
3. Histopathology
   • Core needle biopsy or surgical specimen.
   • Immunohistochemistry: Chromogranin A, synaptophysin, Ki67.
   • Assessment of differentiation (well vs poorly) and grading.
4. Staging follows the AJCC 8th edition TNM system, tailored to each primary site.

Treatment Overview

Management is multidisciplinary and depends on tumour grade, stage, functionality, and patient performance status.

Surgery

• Curative resection is the mainstay for localized disease (R0 resection).
• For midgut NETs, segmental resection with mesenteric lymphadenectomy is standard.
• Even in metastatic disease, debulking (>70% tumour burden) can improve symptoms and may enhance response to systemic therapies.

Somatostatin Analogue (SSA) Therapy

Octreotide LAR and lanreotide are firstline for functional tumours and for disease control in nonfunctional, welldifferentiated NETs (Grade12). They:

• Control hormonerelated symptoms.
• Stabilise tumour growth (PROMID and CLARINET trials).

Targeted Therapies

• Everolimus mTOR inhibitor; approved for progressive NETs of the pancreas, GI tract, and lung.
• Sunitinib oral tyrosinekinase inhibitor; indicated for pancreatic NETs.

Peptide Receptor Radionuclide Therapy (PRRT)

177LuDOTATATE delivers radiation to somatostatinreceptorpositive cells. The NETTER1 trial demonstrated a median progressionfree survival of 28.4months versus 8.4months with highdose octreotide.

Chemotherapy

Primarily reserved for highgrade (G3) NETs and NECs. Regimens include:

• Platinumetoposide for poorly differentiated NECs.
• Capecitabinetemozolomide (CAPTEM) for welldifferentiated G3 NETs.

Management of Functional Syndromes

• SSAs for carcinoid syndrome; add telotristat ethyl to control refractory diarrhoea.
• Protonpump inhibitors for gastrinproducing tumours.
• Diazoxide or somatostatin analogues for insulinomarelated hypoglycaemia.
• Supportive care (fluid replacement, electrolyte correction) for VIPomainduced diarrhoea.

Followup and Prognosis

Longterm followup is essential because recurrences can appear years after initial therapy.

• Lowgrade (G1G2) NETs have 5year survival rates ranging from 70% to >90% when confined to the primary organ.
• G3 NETs and NECs show markedly poorer outcomes, with median overall survival of 1224months for metastatic disease.

Surveillance protocols typically include:

• Clinical assessment and hormone measurements every 36months.
• Crosssectional imaging (CT/MRI) annually, or sooner if symptoms change.
• 68GaDOTATATE PET/CT every 23years in stable disease, more frequently if progression is suspected.

Current Research Directions

Key areas of investigation include:

• Combination PRRT with checkpoint inhibitors early phase trials show encouraging response rates.
• Novel molecular targets (e.g., DLL3, CXCR4) for antibodydrug conjugates.
• Liquid biopsy for circulating tumour DNA to monitor disease dynamics.
• Optimising sequencing of targeted agents versus chemotherapy in G3 NETs.

For personalized recommendations, patients should be evaluated by a multidisciplinary team experienced in neuroendocrine tumour management.