Small bowel carcinoid tumor (neuroendocrine tumor — NET) is the second most common primary malignant tumor of the small intestine, accounting for 25-30% of all small bowel tumors. It arises from enterochromaffin cells (Kulchitsky cells) and can secrete serotonin, chromogranin A, and other bioactive peptides. Most commonly located in the distal ileum. Characteristically shows a small submucosal/intraluminal polypoid lesion with a prominent desmoplastic mesenteric mass — this mesenteric mass is often much larger than the primary tumor with pathognomonic 'sunburst' calcification. Carcinoid syndrome (flushing, diarrhea, bronchospasm, right heart valve disease) develops in the presence of liver metastasis because serotonin enters the systemic circulation without being metabolized by the liver. CT enterography and somatostatin receptor scintigraphy (Octreoscan/Ga-68 DOTATATE PET) are fundamental modalities for diagnosis and staging.
Age Range
40-80
Peak Age
60
Gender
Equal
Prevalence
Uncommon
Small bowel carcinoid tumors arise from submucosal enterochromaffin cells (Kulchitsky cells). These cells are neural crest-derived and secrete serotonin (5-HT), chromogranin A, substance P, and other bioactive peptides. The tumor grows slowly but has a tendency for lymphatic and venous invasion at early stages. The characteristic desmoplastic mesenteric mass results from fibrosis in mesenteric fat tissue caused by tumor-secreted serotonin and growth factors (TGF-β, CTGF) — this desmoplastic reaction can cause narrowing and kinking around mesenteric vessels leading to chronic mesenteric ischemia. Sunburst calcification forms from radial distribution of dystrophic calcification within the desmoplastic mass. Liver metastases are hypervascular because tumor neovascularization is intense — showing prominent arterial phase enhancement. Carcinoid syndrome develops only in the presence of liver metastasis because serotonin passing through portal venous drainage is inactivated by hepatic first-pass metabolism; liver metastasis secretes serotonin directly into hepatic veins entering systemic circulation.
Calcification pattern radiating from a central point within the desmoplastic mesenteric mass. This pathognomonic finding is specific to the desmoplastic reaction of small bowel carcinoid tumor and is not seen in other mesenteric masses. Best evaluated on non-contrast CT.
Prominently enhancing desmoplastic mass in the mesentery on arterial phase — generally 2-5 cm in diameter with spiculated margins, retraction and tethering in surrounding mesenteric fat tissue. 'Sunburst' pattern calcification within the mass — calcification radiating from a central point. This mesenteric mass is generally much larger than the primary ileal tumor and may be noticed before the primary lesion in diagnosis. Mesenteric vascular structures show kinking and narrowing around the mass.
Report Sentence
Desmoplastic mass with sunburst calcification in the mesentery is observed, consistent with small bowel carcinoid tumor (neuroendocrine tumor).
Small (usually 1-2 cm) submucosal/intraluminal polypoid lesion with prominent arterial phase enhancement in the distal ileum. The tumor is hypervascular and enhances more intensely than the surrounding bowel wall. May be multifocal (multiple primaries in 30% of cases). Primary tumor can be small and difficult to detect without the mesenteric desmoplastic mass.
Report Sentence
Small submucosal lesion with prominent arterial phase enhancement in the distal ileum is observed, consistent with carcinoid tumor (NET).
Multiple hypervascular liver metastases in the arterial phase — showing prominent homogeneous or peripheral enhancement. Isodense or slightly hyperdense in portal venous phase. Metastases are generally multiple with bilobar distribution. Carcinoid syndrome (flushing, diarrhea) may develop in the presence of liver metastasis.
Report Sentence
Multiple hypervascular metastases with prominent arterial phase enhancement in the liver are observed, consistent with neuroendocrine tumor metastasis.
Intense somatostatin receptor uptake in the primary tumor, desmoplastic mesenteric mass, and metastases on Ga-68 DOTATATE PET-CT. SUVmax typically reaches very high values of >15-20. This modality is superior in detecting small metastases and multifocal primary tumors undetectable by conventional imaging. Used for patient selection for peptide receptor radionuclide therapy (PRRT — Lu-177 DOTATATE).
Report Sentence
Intense somatostatin receptor uptake in the ileum, mesentery, and liver on Ga-68 DOTATATE PET-CT is observed, consistent with metastatic neuroendocrine tumor.
Kinking, narrowing, and retraction of mesenteric arteries due to desmoplastic reaction on portal venous phase. SMA branches are pulled by the mass creating a 'spoke-wheel' pattern. In advanced cases, mesenteric ischemia findings (bowel wall thickening, pneumatosis) may develop. CTA reconstructions optimally demonstrate vascular involvement.
Report Sentence
Kinking and retraction of mesenteric vascular structures around the desmoplastic mesenteric mass are observed, consistent with carcinoid-associated desmoplastic reaction.
Mesenteric desmoplastic mass shows low to intermediate signal on T2-weighted images — due to fibrous tissue content. Primary ileal tumor appears mildly T2 hyperintense. This contrast difference confirms the fibrous nature of the desmoplastic mass on MRI. On dynamic contrast-enhanced MRI, the mass shows progressive enhancement — slow contrast agent accumulation of fibrous tissue.
Report Sentence
T2 hypointense desmoplastic mass in the mesentery with a small T2 hyperintense tumoral lesion in the adjacent ileum on MRI, consistent with carcinoid tumor and desmoplastic reaction.
Criteria
Mitotic index <2/10 HPF, Ki-67 <3% — well differentiated
Distinct Features
Most common type. Slow growth, high SSTR2 expression, intense DOTATATE PET uptake. Controllable with somatostatin analogs (octreotide/lanreotide). 5-year survival >80%.
Criteria
Mitotic index 2-20/10 HPF, Ki-67 3-20% — well differentiated
Distinct Features
Intermediate aggressiveness, SSTR2 expression usually preserved. May show dual uptake on DOTATATE and FDG PET. Treatment with somatostatin analogs + chemotherapy combination.
Criteria
Mitotic index >20/10 HPF, Ki-67 >20% — poorly differentiated neuroendocrine carcinoma
Distinct Features
Rare in small bowel. Aggressive behavior, SSTR2 expression low/lost → decreased DOTATATE uptake, FDG PET more sensitive. Chemotherapy (platinum-based) is primary treatment.
Distinguishing Feature
Adenocarcinoma shows annular wall thickening and luminal obstruction; carcinoid is characterized by small submucosal lesion + large desmoplastic mesenteric mass
Distinguishing Feature
Lymphoma shows diffuse wall thickening and aneurysmal dilation, calcification rare; in carcinoid, sunburst calcification and hypervascular enhancement are pathognomonic
Distinguishing Feature
GIST shows exophytic growth and heterogeneous enhancement (central necrosis); carcinoid shows intramural/submucosal small lesion and mesenteric desmoplasia
Distinguishing Feature
Metastasis typically presents with multiple lesions and known primary malignancy; carcinoid has solitary or few primaries + pathognomonic desmoplastic mesenteric mass
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
specialist-referralTreatment of small bowel carcinoid tumor requires a multidisciplinary approach. Primary tumor and desmoplastic mass are surgically resected — surgery may be complex due to vascular involvement. For liver metastases, somatostatin analogs (octreotide LAR, lanreotide) are the primary medical treatment — slowing tumor growth and controlling carcinoid symptoms. Peptide receptor radionuclide therapy (PRRT — Lu-177 DOTATATE) is effective in progressive disease. Everolimus (mTOR inhibitor) is second-line treatment. Chromogranin A and 5-HIAA (urine) are follow-up markers. Staging is performed with Ga-68 DOTATATE PET-CT.
Carcinoid tumors are slow-growing neuroendocrine tumors with metastatic potential. Carcinoid syndrome (flushing, diarrhea, bronchospasm) occurs with liver metastases. Ga-68 DOTATATE PET-CT provides somatostatin receptor imaging for staging.