Etiology and Pathogenesis of Small Bowel Adenocarcinoma

• Small bowel tumors are relatively rare:
  • Comprising only:
    • 5% of all gastrointestinal malignancies
• Despite the small intestine being:
  • 75% to 80% of the total length of the gastrointestinal tract:
    • Small bowel adenocarcinomas are less common:
      • Than colorectal adenocarcinomas
        • This may be due:
          • To the more dilute and liquid small bowel contents
          • More rapid transit
          • Decreased bacterial load (compared to the colon):
            • Although the precise reasons for this low rate of malignancy are unknown
• Estimated new cases and deaths from small intestine cancer in the United States in 2020:
  • New cases:
    • 11,110
  • Deaths:
    • 1,700
• Adenocarcinoma (36.9%), lymphoma (17.3%), sarcoma (8.4%), and carcinoid tumors (37.4%):
  • Account for the majority of small intestine malignancies:
    • Which, as a whole:
      • Account for only 4% of all gastrointestinal malignancies
• Small bowel adenocarcinomas:
  • Comprise approximately 37% of all small bowel malignancies:
    • And are the second most common small bowel cancer:
      • Slightly less common than small bowel carcinoid tumors (37.4%):
        • 9.3 per million new cases of small bowel carcinoid tumor annually in the United States
• There are an estimated:
  • 7.3 per million new cases of small bowel adenocarcinoma annually in the United States
• Small bowel adenocarcinomas:
  • Arise in men slightly more often than in women:
    • 53% versus 47%
  • The median age at diagnosis:
    • Is 67 years old

• Small bowel adenocarcinomas:
  • Are thought to arise from adenomas:
    • But the details of this process are less well understood:
      • Than the natural history and biology of colorectal adenoma to adenocarcinoma development
  • There are several known risk factors for small bowel adenocarcinoma, including:
    • Familial syndromes:
      • Lynch syndrome
      • Familial adenomatous polyposis [FAP]
      • Peutz-Jeghers syndrome
    • Chronic inflammation:
      • Crohn disease
      • Celiac disease
    • Previous colorectal adenocarcinoma.

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