Diagnostic Approach to a Suspected Small Bowel Neuroendocrine Tumor

👉Patients with small bowel carcinoids often present with a mesenteric mass without an imageable small bowel primary tumor.

• The differential diagnosis of an isolated mesenteric tumor includes:
  • Lymphoma
  • Desmoid tumor
  • Reactive lymphadenopathy:
    • From an inflammatory process
  • Mesenteric peritoneal implant:
    • From an abdominal malignancy
  • Small bowel carcinoid
• Desmoid tumors:
  • Often have a spiculated appearance on CT
• In carcinoid tumors with associated mesenteric masses:
  • The relationship to the major mesenteric vessels should be assessed:
    • As nodal carcinoid metastases:
      • Can be unresectable:
        • If they involve the root of the mesentery
• Patients with mesenteric masses should undergo:
  • Biochemical testing for carcinoid:
    • Serum chromogranin A
    • Urine 5-hydroxyindoleacetic acid [5-HIAA])
  • Endoscopy:
    • If the small bowel associated with the mesenteric mass is endoscopically accessible
  • Small bowel enterography (CT or MRI):
    • Can occasionally identify a previously occult primary small bowel tumor

Chromogranin A

• Chromogranins are peptides:
  • Released from neuroendocrine cells:
    • Vary day to day and with food intake
• Depending on the threshold used:
  • The sensitivity approaches 95%
  • The specificity is low (55%):
    • Given the high rate of false positivity:
      • As it is elevated in multiple other conditions, including:
        • Endocrine diseases
        • Inflammatory conditions
        • Proton pump inhibitor use

5-HIAA

• 5-hydroxyindoleacetic acid (5-HIAA):
  • Is the end product of serotonin metabolism:
    • It is excreted in the urine
• Twenty-four-hour urinary excretion of 5-HIAA:
  • Can be elevated:
    • In patients with carcinoid tumors
  • It is most useful in patients:
    • With carcinoid syndrome:
      • Where it has high (90%) sensitivity and specificity
  • In patients with carcinoid tumors:
    • Without carcinoid syndrome:
      • The sensitivity is lower (~ 70%) even when using a low-level 5-HIAA cutoff
• Foods containing high levels of tryptophan or serotonin and certain drugs:
  • Can result in false positive values

CT

• Small bowel carcinoid tumors:
  • Are often small (less than 2 cm median size):
    • And are thus difficult to identify by cross-sectional imaging
• Given the hypervascularity of these tumors:
  • Arterial phase:
    • May improve visibility
• More commonly:
  • CT imaging reveals:
    • Mesenteric or hepatic metastases:
      • Without a small bowel mass
• The classic CT appearance demonstrates:
  • A “spokes in a wheel” pattern:
    • With a mesenteric nodal mass (wheel)
    • With radiating desmoplastic fibrosis
  • The occult primary small bowel carcinoid tumor:
    • Is often in the bowel adjacent to the nodal metastases:
      • And may manifest with radiographic signs of:
        • A partial small bowel obstruction
• Enterography (CT or MR):
  • May have higher sensitivity in detection small bowel carcinoids:
    • But is not universally available and is understudied
  • CT imaging:
    • Often underestimates the extent:
      • Of mesenteric, peritoneal, and hepatic metastases

OctreoScan

• Indium-111 pentetreotide (OctreoScan):
  • Exploits the presence of:
    • Somatostatin receptors on carcinoid tumor cells:
      • Unlike high-grade neuroendocrine:
        • Low-grade carcinoid tumors:
          • Express:
            • High levels of somatostatin receptors
• Octreotide scans:
  • Can allow for metastatic assessment and can predict response to somatostatin analogue therapy
    • However:
      • The spatial resolution and sensitivity of small carcinoid tumor detection:
        • Is limited
• Functional PET/CT (i.e., gallium-68 dotatate) scans:
  • Offer improved sensitivity and resolution:
    • And are preferred where available

ENDOSCOPY

• Similar to small bowel adenocarcinomas:
  • Small bowel carcinoids must be in an endoscopically accessible location to be visualized by endoscopy
  • Because they are often in the distal most 60 cm of the terminal ileum:
    • Colonoscopy or retrograde enteroscopy:
      • Can often reach these tumors
• Endoscopic assessment allows opportunities to:
  • Biopsy and tattoo the lesion for identification during resection

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