Small bowel tumours and GIST

• Cancer of the small bowel is rare—accounting for under 5% of gastrointestinal cancers—yet its incidence is rising everywhere, and adenocarcinoma, neuroendocrine tumour, GIST and lymphoma each need a different plan.^[1]
• Presentation is usually vague: abdominal pain, bleeding, anaemia, weight loss, obstruction, perforation, or—in neuroendocrine disease—flushing and diarrhoea; many cases are found only by capsule or balloon enteroscopy.^[1][2]
• GIST is the mesenchymal small-bowel tumour that proved the power of molecular medicine: about 70% carry KIT mutations and 10–15% PDGFRA mutations, and imatinib is both the standard adjuvant therapy and first-line treatment for unresectable or metastatic disease.^[3]
• Curative treatment is surgical resection whenever possible; adjuvant therapy follows for GIST (imatinib) and high-risk resected adenocarcinoma (fluoropyrimidine-based).^[4][5]
• Five-year survival ranges from roughly 20–40% for adenocarcinoma to over 70% for GIST and localized neuroendocrine tumour, so naming the histology early changes everything.^[1][2]

By the end you should be able to:

1. Name the four common malignant small-bowel tumours, their typical sites and frequencies.^[1][2]
2. Explain symptoms and signs from mechanism, with the particular features of obstruction and bleeding.^[2]
3. Plan investigation of suspected small-bowel tumour bleeding, from CT or MR enterography through capsule and balloon enteroscopy.^[1][2]
4. State the KIT/PDGFRA basis of GIST, the mutation tests and the imatinib indications.^[3][4]
5. Apply GIST risk stratification—size, site, mitotic count, rupture—to decide adjuvant imatinib.^[3][4]
6. Outline treatment of adenocarcinoma, lymphoma, neuroendocrine tumour and metastatic tumour.^[5][6]

A small-bowel tumour is any neoplasm of the duodenum, jejunum or ileum. Clinically we separate mucosal tumours (adenocarcinoma, neuroendocrine tumour, lymphoma) from wall (mesenchymal) tumours (GIST, sarcoma) and from deposits that have spread to the bowel from elsewhere (metastases).^[1][2] Most are epithelial, and of these most are adenocarcinomas.^[1]

Four groups dominate:

• Adenocarcinoma—a gland-forming mucosal malignancy, usually duodenal or proximal jejunal, behaving like colorectal cancer.^[1]
• Neuroendocrine tumour—a well-differentiated tumour of enterochromaffin cells, usually ileal, often slow-growing and hormone-active.^[1][6]
• GIST—the commonest mesenchymal small-bowel wall tumour, arising from interstitial cells of Cajal and driven by KIT or PDGFRA mutation.^[3]
• Lymphoma and metastasis—non-Hodgkin lymphoma (usually DLBCL or MALT in the ileum) and secondary deposits, most often from melanoma, lung, colon, breast or kidney.^[1][2]

Benign lesions are commoner overall and cause identical symptoms; telling benign from malignant is a routine step.^[2]

The small bowel is nearly three-quarters of the gut by length and over 90% of its mucosal surface, yet malignant tumours here are under 5% of gastrointestinal cancers.^[1] Incidence has more than doubled in four decades, from roughly 1.0 to over 2.0 per 100,000, driven mainly by adenocarcinoma and neuroendocrine tumour; peak age is the mid-sixties with a slight male excess.^[1] Five-year outcomes vary sharply: adenocarcinoma 20–40%, GIST over 70%, localized neuroendocrine tumour over 80%.^{[1][2] [3]}

Western series contain more neuroendocrine tumour (30–40% vs 10–20%) and more Crohn's-related adenocarcinoma (15–30% vs under 5%) than Asian series.^[1] GIST occurs overall at 10–15 cases per million per year; 20–25% arise in the small bowel and 60–70% in the stomach, including 2.2% of all malignant gastric tumours.^{[3] [2]}

Most small-bowel tumours are sporadic. Recognisable predispositions raise specific diagnoses: Crohn's disease (up to 30% of small-bowel adenocarcinomas in the West), coeliac disease (enteropathy-associated T-cell lymphoma, rare in Asia), inherited syndromes (familial adenomatous polyposis or Lynch syndrome for adenocarcinoma; neurofibromatosis type 1 for multifocal GIST; SDH-deficient and Carney-complex–type syndromes for young patients), chronic intestinal inflammation, immunosuppression and previous radiotherapy (lymphoma).^[1][3]

For GIST the driving events are somatic KIT (about 70% of tumours) or PDGFRA (10–15%) mutations, with smaller groups linked to NF1 (9%), SDH deficiency (7%), BRAF (3%) and rare fusions. KIT/PDGFRA mutation and SDH deficiency are mutually exclusive, and mutation testing predicts imatinib response.^[3]

Three mechanisms dominate: luminal narrowing, bleeding, and hormone secretion.

Obstruction. The small bowel is narrow and peristalsis is vigorous, so even modest thickening can obstruct. Lymphoma and GIST often present in the distal ileum with obstruction, intussusception or perforation; duodenal disease causes vomiting and early satiety; functional neuroendocrine tumours thicken bowel wall and mesentery through serotonin-driven fibrosis, fixing and kinking loops.^[2][6]

Bleeding. Tumour neovascularity and surface ulceration produce bleeding that is occult for months, presenting as iron-deficiency anaemia, and only later as melaena or overt haemorrhage. Jejunal tumours bleed more readily than ileal tumours. Obscure gastrointestinal bleeding is the second commonest presentation of small-bowel tumour disease, and tumours are found in up to 8.9% of patients with iron-deficiency anaemia after negative bidirectional endoscopy in pooled series.^[2]

Hormone secretion. Ileal neuroendocrine tumours release serotonin and other mediators. When metastases bypass hepatic clearance, carcinoid syndrome results—flushing, secretory diarrhoea, wheeze and right-sided valvular heart disease—plus tryptophan diversion and a pellagra-type deficiency. Serotonin metabolites also cause mesenteric and cardiac fibrosis, making this a systemic disease.^[6]

GIST biology. Mutated KIT/PDGFRA receptors signal continuously through MAPK, PI3K-AKT-mTOR and JAK-STAT, driving proliferation and survival. This explains why imatinib works and why response depends on the mutation site.^[3]

Adenocarcinomas resemble colorectal tumours (KRAS/TP53 mutations predominate; microsatellite instability in under 10%).^[1] Neuroendocrine tumours are graded by mitoses and Ki-67; they often metastasise despite bland appearances.^[1][6] Lymphomas follow the WHO scheme (DLBCL, MALT, follicular, mantle cell, enteropathy-associated T-cell).^[1] GIST is typically a monotonous spindle-cell tumour; epithelioid morphology occurs in only 3–5% of small-bowel GISTs. Diagnosis pairs CD117 (c-kit) and/or DOG1 with mutation testing. Most GISTs are localized at diagnosis (80–90%), but 10–20% present with metastases and a localized GIST resected alone carries a 10-year recurrence-free survival of 63%.^[3]

Typical complaints: intermittent colicky pain; bilious vomiting; bloating; black or tarry stools or visible bleeding; fatigue and breathlessness from anaemia; weight loss; secretory diarrhoea in carcinoid syndrome; flushing, wheeze or palpitation; a palpable lump (uncommon); or no symptoms at all until incidental imaging. Obstruction, bleeding, invagination and perforation are frequent—emergency surgery was needed in 17.6% in one series.^[1][2]

Anaemia, cachexia, an abdominal mass, hepatomegaly from metastases (prominent in neuroendocrine disease), obstructive distension, right-sided cardiac murmur with carcinoid syndrome, skin flushing, or telangiectasia. Duodenal disease may mimic gastric outlet obstruction. Small-bowel GIST is often an extraluminal mass on imaging.^[3]

Most common distally. Lymphoma offers a lead point for intussusception; GIST and adenocarcinoma narrow the lumen; neuroendocrine tumours fix loops by fibrosis. Perforation signals aggressive or necrotic tumour. Neuroendocrine tumours need emergency resection for obstruction or perforation in roughly 8% of cases.^[2][6]

Bleeding is often the first clue and is the second commonest cause of obscure gastrointestinal bleeding; it may be overt or occult. Jejunal tumours bleed more often than ileal tumours. GIST can ulcerate a submucosal mass and bleed torrentially; neuroendocrine tumours may bleed from mucosal ulceration or mesenteric traction.^[2][3]

Pain, bleeding, anaemia and obstruction are shared by many conditions, so imaging and biopsy are essential.

The key question is whether a stricture or mass is malignant or inflammatory, and primary or metastatic; biopsy settles it.^[2]

Iron-deficiency anaemia, hypoalbuminaemia and weight loss raise suspicion; liver tests and lactate dehydrogenase help gauge metastatic load. In neuroendocrine disease measure urinary 5-hydroxyindoleacetic acid (and in selected centres chromogranin A). Histology is always required.^[1][6]

CT enterography (CTE) is the usual first test (about 80% sensitivity for mass-forming lesions) and assesses nodes and metastases. MR enterography matches CTE without radiation and is favoured in younger patients. CT and MR guide decisions about resectability and nodal dissection.^[1][2]

Capsule endoscopy is non-invasive, visualises the whole mucosa and detects small-bowel tumour in 50–70% of cases, but it cannot biopsy. Device-assisted enteroscopy (double- or single-balloon or spiral) provides vision, biopsy and haemostasis, with 60–80% yield. Ileocolonoscopy assesses the terminal ileum. A capsule-then-balloon strategy is the modern standard for obscure-bleeding tumours. ^[1]

Small-bowel adenocarcinoma is staged by the TNM system (depth of invasion, nodes, metastases) and by the total number of nodes examined. Neuroendocrine tumours use WHO grade (G1, G2, G3) derived from mitotic count and Ki-67, plus TNM stage. For GIST, staging incorporates site, size, mitotic rate and tumour rupture.^[3][6]

Testing for KIT and PDGFRA mutations is recommended before any tyrosine kinase inhibitor is contemplated, because the mutation site predicts response. KIT mutations occur in about 70% of GISTs, most often in exon 11 (around 65%) and exon 9 (around 10%); PDGFRA mutations occur in 10–15%, mostly in exon 18 (around 6%).^[3]

Risk of recurrence after complete resection depends on size, site and mitotic count; tumour rupture behaves like metastatic disease. Mitotic count is now expressed per area, not per field: WHO 5th edition, ESMO-EURACAN-GENTURIS 2022 and NCCN/CAP 2025 all define a high mitotic rate as more than 5 mitoses in 5 mm2 (approximately 20–25 high-power fields with modern microscopes). This corrects the older "per 50 HPF" systems, in which the examined area varied from 4.8 mm2 to 11.9 mm2 with different microscope eyepieces.^[3]

Two widely used systems are:

• NIH consensus (2002)—very low, low, intermediate or high risk from size and mitotic count per 50 high-power fields.^[3]
• AFIP (Miettinen and Lasota, 2006)—adds site: small-bowel GIST carries a higher risk than gastric GIST for the same size and mitotic count. For example, small-bowel GIST of 5–10 cm with 5 or fewer mitoses per 50 high-power fields carries about a 24% risk of progressive disease, against about 3.6% for the same gastric lesion; a very small small-bowel GIST with more than 5 mitoses carries about 50% risk.^[3]

Small-bowel GIST is therefore, in practice, treated more aggressively than gastric GIST of identical dimensions. Tumour rupture occurs in 10–27% of high-risk GISTs and is followed by a recurrence rate of about 68% irrespective of adjuvant treatment; NCCN equates rupture with metastatic disease.^[4]

• Obstruction: nil by mouth, intravenous fluids, nasogastric decompression, broad-spectrum antibiotics if strangulation or perforation is suspected, and surgery without delay.^[2]
• Bleeding: resuscitate, transfuse according to haemoglobin, attempt endoscopic haemostasis when a lesion is reachable, and operate if unstable; bleeding GIST can be torrential.^[2][3]
• Perforation or sepsis: urgent surgery.^[2]
• Carcinoid crisis: patients with carcinoid syndrome need pre-operative somatostatin analogue infusion and cautious anaesthesia, because manipulation can precipitate flushing, bronchospasm and hypotension.^[6]

Adenocarcinoma. Curative treatment is surgical resection with regional lymphadenectomy (segmental resection or pancreaticoduodenectomy for duodenal tumours); duodenal adenocarcinoma has worse survival than jejunal or ileal disease. NCCN recommends 3–6 months of fluoropyrimidine-based adjuvant chemotherapy for node-positive duodenal adenocarcinoma or tumours with high-risk features (positive margins, fewer than 5 lymph nodes examined, perforation, lymphovascular invasion, perineural invasion, poorly differentiated histology). There are no completed prospective adjuvant trials—the standard is modelled on colorectal practice—so these decisions should be reviewed in a multidisciplinary team. Metastatic small-bowel adenocarcinoma is treated with fluoropyrimidine- and platinum-based combinations.^[2][5]

Lymphoma. Primary small-bowel lymphoma is staged and then treated as systemic non-Hodgkin lymphoma: chemotherapy with rituximab for B-cell types, with surgery reserved for complications (obstruction, perforation, bleeding) and for selected localized disease. Many patients are managed without resection. Tissue diagnosis and subtyping must be completed before treatment.^[1][2]

Neuroendocrine tumour. Segmental resection with en bloc mesenteric lymphadenectomy is the cornerstone, with careful intraoperative search for multifocal disease and mesenteric plaques. The primary is usually resected even when metastases are present, because of the risk of obstruction. Prophylactic early surgery for asymptomatic stage IV disease has not shown a survival advantage over delayed surgery in a European cohort and caused more re-operations. Somatostatin analogues (octreotide long-acting release or lanreotide) are first-line for advanced or unresectable functional disease: PROMID (85 patients, octreotide vs placebo) and CLARINET (204 patients with Ki-67 under 10%, lanreotide vs placebo) both showed improved progression-free survival. Current guidelines do not recommend routine adjuvant systemic treatment after R0/R1 resection of well-differentiated disease; cytotoxic chemotherapy, everolimus and PRRT are used for progressive disease.^[6]

GIST. Surgical resection is the only potentially curative treatment and should be performed as soon as work-up allows, because behaviour cannot be predicted from size or consistency. Complete resection with an intact pseudocapsule is the goal; tumour rupture converts the outlook to metastatic risk.^[3][4]

Adjuvant imatinib: three years of imatinib (400 mg once daily) is the standard of care for high-risk GIST after complete resection; extension beyond 3 years may be considered in very high-risk patients but is not universally recommended. Low-risk disease does not benefit. Mutational testing is mandatory before starting, because KIT exon 11 tumours are most sensitive. Ruptured GIST carries around 68% recurrence despite adjuvant treatment. The ESMO and NCCN guidelines both recommend 3 years but differ slightly on individualized extension. Imatinib 800 mg daily is established in metastatic disease, particularly for KIT exon 9 tumours, but is not routinely used in the adjuvant setting.^[3][4]

Imatinib-resistant subtypes: GIST with PDGFRA D842V is treated with avapritinib; SDH-deficient GIST is usually imatinib-resistant and is managed by surgery, observation, sunitinib or regorafenib; NF1-associated GIST has no established systemic therapy.^[3]

Advanced/metastatic GIST: imatinib first-line (400 mg; 800 mg for exon 9), sunitinib second line, regorafenib third line, avapritinib for PDGFRA D842V, ripretinib after several prior lines. Resection of selected metastases can be considered in a multidisciplinary discussion.^[3]

Metastases to the small bowel are treated as metastatic disease of their primary. Palliative resection is reserved for obstruction, haemorrhage, diagnostic uncertainty or selected cytoreduction. In one 7-year series, 10.3% of malignant small-bowel lesions were metastases, from melanoma, urothelial carcinoma and lung cancer.^[2]

Nutritional rehabilitation and iron replacement are routine. Somatostatin analogues control carcinoid syndrome but do not cure it. Postoperative follow-up combines clinical review, imaging and (when indicated) endoscopy.^[1][2]

High-risk GIST can recur after stopping imatinib, so regular imaging is needed; very high-risk patients may need prolonged therapy. Neuroendocrine follow-up is guided by recurrence-risk models (ENETS, NCCN, ESMO, NANETS) with individualised surveillance intervals. Adenocarcinoma surveillance follows colorectal-style protocols.^[4][6]

Refer urgently—same day if bleeding or obstructed—to gastroenterology, surgery and oncology. Early referral matters because definitive treatment is surgical and because GIST needs mutation testing and specialist oncology before any tyrosine kinase inhibitor is started. Rarer disease is best managed in a multidisciplinary GIST/sarcoma or neuroendocrine team.^[2][3][4]

• Obstruction and perforation are the commonest acute presentations; emergency surgery is often required.^[2]
• Invagination/intussusception can be caused by ileal lymphoma or a pedunculated GIST acting as a lead point.^[2]
• Haemorrhage, sometimes torrential, from neuroendocrine or GIST lesions.^[2][3]
• Metastatic spread to liver and peritoneum, especially in neuroendocrine and adenocarcinoma disease.^[2]
• Carcinoid heart disease—right-sided valvular fibrosis from serotonin metabolites.^[6]
• Treatment complications—postoperative bleeding, ileus, infection; tyrosine kinase inhibitor side-effects; carcinoid crisis with anaesthesia.^[3][6]

Survival is histology-driven: adenocarcinoma 20–40%, GIST over 70%, localized neuroendocrine tumour over 80%, and lymphoma 50–60% depending on subtype. GIST outcomes are further modified by mitotic index, site and rupture; adenocarcinoma by nodal yield and stage; neuroendocrine disease by grade and metastasis. Cure falls sharply with metastatic adenocarcinoma, although GIST and neuroendocrine tumours can run long courses with targeted therapy.^[1][2][3][6]

There is no population screening for small-bowel tumours. Prevention means controlling predisposing disease (Crohn's, coeliac), surveillance of polyps in familial adenomatous polyposis or Lynch syndrome with enteroscopy or capsule endoscopy in some centres, and prompt investigation of iron-deficiency anaemia or obscure bleeding rather than late.^[1][2]

• Children: rare; lymphoma (including Burkitt) and polyposis syndromes dominate; presentation is often as intussusception.^[1]
• Pregnancy: manage any malignant case with a multidisciplinary team; surgery remains the cornerstone.
• Older adults: adenocarcinoma predominates; fitness, not age alone, dictates surgical candidacy.^[2]
• Immunocompromised (transplant, HIV, long-term immunosuppression): lymphoma risk is higher; exclude tuberculosis before labelling a lesion malignant.^[1]

Capsule endoscopy is not essential: CT enterography, careful diagnostic laparoscopy and judicious open surgery diagnose most cases. GIST can be identified with CD117 or DOG1 staining; where molecular testing is unavailable, management relies on size, mitotic count and empirical imatinib in selected advanced cases, at the cost of mutation-directed choices (such as avapritinib for PDGFRA D842V). Obscure bleeding with negative upper and lower endoscopy should still prompt a search for a small-bowel tumour. Centralising rare-tumour care in regional oncology centres improves outcomes.^[2][3]

• Small-bowel cancer is rare (under 5% of gastrointestinal cancers) but rising; four histologies dominate: adenocarcinoma, neuroendocrine tumour, GIST and lymphoma.^[1]
• Adenocarcinoma is commonest (40–60% in Asian series), usually duodenal/proximal jejunal, with the worst survival (20–40%).^[1]
• Neuroendocrine tumours favour the ileum; localized disease carries over 80% five-year survival; somatostatin analogues are first-line for advanced functional disease (PROMID, CLARINET).^[1][6]
• GIST arises from interstitial cells of Cajal, is driven by KIT or PDGFRA mutations, and is diagnosed by CD117/DOG1 plus mutation testing.^[3]
• Imatinib is standard adjuvant therapy and first-line for unresectable/metastatic GIST; 400 mg daily for 3 years post-resection is standard for high risk.^[3][4]
• Small-bowel GIST is more aggressive than gastric GIST; site, size, mitotic count (now more than 5 mitoses in 5 mm2 defines high rate) and rupture all matter; rupture behaves like metastatic disease.^[3][4]
• Obscure gastrointestinal bleeding or unexplained iron-deficiency anaemia should lead to CT/MR enterography and capsule endoscopy; DAE obtains the biopsy.^[1][2]
• Surgical resection is the only curative treatment for small-bowel tumours; adjuvant fluoropyrimidine chemotherapy is standard for high-risk resected adenocarcinoma.^[2][5]
• Complications such as obstruction, perforation and torrential bleeding are common; refer early.^[2]
• Survival is histology-driven; confirm the type quickly.^[1][2]

Small bowel obstruction; obscure gastrointestinal bleeding and iron-deficiency anaemia; colorectal adenocarcinoma; gastric cancer; sarcoma; neuroendocrine tumours; capsule and balloon enteroscopy; oncology multidisciplinary care