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Surgical Pathology Criteria

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Rectal Carcinoid / Well Differentiated Neuroendocrine Neoplasm / Tumor


  • Low grade neoplasm of the the rectum and distal colon demonstrating neuroendocrine differentiation


Alternate/historical Names

  • Hindgut carcinoid
  • Well differentiated (neuro-)endocrine tumor/carcinoma are equivalent terms

Diagnostic Criteria

  • Most large intestine carcinoids arise in the rectum with smaller numbers in the descending and sigmoid colon
    • Most others are cecal, from near the ileocecal valve
      • Such neoplasms share histopathologic and clinical features with ileal carcinoids
  • Most cells show evidence of neuroendocrine differentiation
    • Synaptophysin stain positive in nearly all cases
      • Chromogranin frequently negative in rectal carcinoids
    • Argyrophil stain positive in nearly all cases
      • Argentaffin frequently negative in rectal carcinoids
    • Neuron specific enolase, PGP9.5 and CD56 are sensitive but not specific
  • Most well differentiated neoplasms exhibit characteristic cytologic and architectural features
    • Round regular nuclei
      • Stippled (salt and pepper) chromatin
    • Moderate to abundant cytoplasm
    • Various growth patterns
      • Insular growth pattern
        • Round nests of cells
        • May palisade
      • Trabecular
        • Rows and strands of cells
        • Cells are “stacked” with their long axis perpendicular to the long axis of the row
      • Tubular
        • Lined by single layer of uniform cells
        • True glands with intracytoplasmic mucin not seen
      • May form rosette-like pseudo-glandular structures
        • True glands with intracytoplasmic mucin not seen
    • Usually involves only base of crypts, leaving mucosa largely intact
  • Carcinoids can be subdivided by pattern and secretory products
    • L cell glucagon-like peptide and pancreatic polypeptide (PP/PYY) producing carcinoid is the dominant pattern in the rectum
      • Predominantly tubular and/or trabecular
      • Most produce GLP-1, GLP-2, glycentin, oxyntomodulin, PP/PYY
      • Serotonin may be positive (30%)
      • Synaptophysin positive, may be chromogranin A negative
        • Chromogranin B positive (not detected by most common anti-chromogranin antibodies)
      • Argyrophil positive, may be argentaffin negative
      • Prostatic acid phosphatase positive in >80% of cases
    • Enterochromaffin (EC) cell serotonin producing carcinoid is uncommon in the rectum
      • Predominantly insular
        • May have S100 positive sustentacular cells around nests (16%)
      • Produce serotonin and substance P
      • Synaptophysin and chromogranin A positive
      • Argentaffin and argyrophil positive
      • Frequent invasion
        • Muscularis propria and serosa
        • May elicit desmoplastic response

    Robert V Rouse MD
    Department of Pathology
    Stanford University School of Medicine
    Stanford CA 94305-5342

    Original posting / updates: 7/27/10, 12/28/11


Differential Diagnosis

  • Conventional GI adenocarcinoma
  • Site of origin of metastatic well differentiated endocrine carcinoma
Rectal Carcinoid / Well Differentiated Neuroendocrine Cell Neoplasm Conventional Colorectal Adenocarcinoma
Bland cytologic features Atypical cytology
Chromogranin or synaptophysin positive Chromogranin and synaptophysin negative (scattered cells OK)
Rare mitotic figures Mitotic figures frequently numerous
Cohesive nests and cords Frequent single cell infiltration
No surface adenoma or in situ lesion May have surface adenoma or in situ component
No intracytoplasmic mucin May have intracytoplasmic mucin


Determination of common sites of origin for metastatic well differentiated endocrine neoplasms
Lung 40-50% Negative Negative Negative Negative
Pancreas Negative 0-18% 28% 68% 50-67%
Stomach Negative 0-17% 60% Negative 20%
Duodenum Negative 0-17% 60% Negative 100%
Ileum Negative >90% Negative Negative Negative
Appendix Negative >90% 55% Negative 21%
Rectum Negative 0-55% 17% Negative 85%
Detection of specific islet hormones may be useful; PAX8 data has been questioned (see Pancreas).


Determination of common sites of origin for metastatic well differentiated endocrine neoplasms-keratins
  CK7 CK20
Lung Variable Negative
Pancreas Variable Variable
Stomach Variable Negative
Duodenum Negative Variable
Ileum Negative Variable
Appendix Negative Variable
Rectum Variable Variable
CK7/20 staining is only helpful in instances of positivity where a negative result is expected


  • Five year survival for rectal carcinoid depends upon extent at presentation
    • Localized at presentation 81%
    • Regional spread at presentation 47%
    • Liver metastases at presentation 18%
    • Deaths due to disease continue to occur beyond 5 years
  • Carcinoid syndrome is rare even if the liver is involved by metastases
  • 13% of patients also have a non-endocrine neoplasm, usually gastrointestinal adenocarcinoma
  • Histopathologic features are predictive of behavior
    • Benign
      • Non-functioning
      • Well differentiated
      • ≤1 cm in greatest dimension
      • Confined to mucosa and submucosa
        • No involvement of muscularis propria
      • No vascular invasion
    • Uncertain malignant behavior
      • As for benign, but with:
        • >1 but ≤2 cm in greatest dimension and/or
        • Vascular invasion
    • Low grade malignant
      • Non-functioning and well differentiated, with
        • Invasion of muscularis propria or beyond and/or
        • Metastasis and/or
        • >2 cm in greatest dimension (not uniformly accepted)
      • OR, functioning and well differentiated
        • Any size and extent

Grading and Staging

  • WHO 2010 recommends the following grading scheme

      Proposed grading scale based on proliferation

      Grade Mitotic count per 10 hpf % of cells Ki67+
      G1 <2 ≤2
      G2 2-20 3-20
      G3 >20 >20
      • Mitotic counts based on 50 hpf
      • Ki67 % based on 500-2000 cells
      • If discrepant, use higher grade
      • Scale proposed by European Neuroendocrine Society (ENETS)
        • Carcinoids / neuroendocrine tumors may be G1 or G2
        • G3 is definitional for high grade neuroendocrine carcinoma
      • Progression from G1/2 to G3 is quite rare
        • G3 appears to be a separate process

      Grading/Staging References

      • Rindi G, Klöppel G, Couvelard A, Komminoth P, Körner M, Lopes JM, McNicol AM, Nilsson O, Perren A, Scarpa A, Scoazec JY, Wiedenmann B. TNM staging of midgut and hindgut (neuro) endocrine tumors: a consensus proposal including a grading system. Virchows Arch. 2007 Oct;451(4):757-62.
      • Rindi G, Klöppel G, Alhman H, Caplin M, Couvelard A, de Herder WW, Erikssson B, Falchetti A, Falconi M, Komminoth P, Körner M, Lopes JM, McNicol AM, Nilsson O, Perren A, Scarpa A, Scoazec JY, Wiedenmann B; and all other Frascati Consensus Conference participants; European Neuroendocrine Tumor Society (ENETS). TNM staging of foregut (neuro)endocrine tumors: a consensus proposal including a grading system. Virchows Arch. 2006 Oct;449(4):395-401.
    • Other features described above should also be reported
  • High grade / poorly differentiated neuroendocrine carcinoma is covered separately
  • AJCC TNM 7th edition now includes carcinoids of the ileum



Gastrointestinal Endocrine Cell Proliferations and Neoplasms



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  • Lin X, Saad RS, Luckasevic TM, Silverman JF, Liu Y. Diagnostic value of CDX-2 and TTF-1 expressions in separating metastatic neuroendocrine neoplasms of unknown origin. Appl Immunohistochem Mol Morphol. 2007 Dec;15(4):407-14.
  • Schmitt AM, Riniker F, Anlauf M, Schmid S, Soltermann A, Moch H, Heitz PU, Klöppel G, Komminoth P, Perren A. Islet 1 (Isl1) expression is a reliable marker for pancreatic endocrine tumors and their metastases. Am J Surg Pathol. 2008 Mar;32(3):420-5.
  • Moskaluk CA, Zhang H, Powell SM, Cerilli LA, Hampton GM, Frierson HF Jr.  Cdx2 protein expression in normal and malignant human tissues: an immunohistochemical survey using tissue microarrays. Mod Pathol. 2003 Sep;16(9):913-9.
  • Srivastava A, Hornick JL. Immunohistochemical staining for CDX-2, PDX-1, NESP-55, and TTF-1 can help distinguish gastrointestinal carcinoid tumors from pancreatic endocrine and pulmonary carcinoid tumors. Am J Surg Pathol. 2009 Apr;33(4):626-32.
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