Colon Polyps

Colon polyps are growths of mucosal tissue in the colon, the most common site of polyps in the GI tract. Polyps can be classified as neoplastic or nonneoplastic and may be associated with genetic syndromes. Hyperplastic polyps are nonneoplastic and are the most common type overall, whereas adenomas are the most common type of neoplastic polyp and have the potential to progress to cancer. For most people without hereditary syndromes, colon cancer screening should begin at age 50 (at age 45 for Black individuals) and in adolescence for those with the rare familial adenomatous polyposis (FAP) syndrome. Diagnosis is by biopsy, and management includes frequent surveillance in people with adenomatous polyps or screening every 10 years in the general population until age 75.

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Overview

Types of colon polyps

  • Benign:
    • Hyperplastic: no neoplastic potential
    • Hamartomatous: mixture of normal tissues but growing in a disorganized mass
      • Juvenile: usually solitary, can be seen in children, not associated with an increased risk of colorectal cancer (CRC)
      • Phosphatase and tensin homolog (PTEN) hamartoma tumor syndrome: comprised of the Cowden and Bannayan-Riley-Ruvalcaba syndromes
      • Peutz-Jeghers polyps: usually associated with Peutz-Jeghers syndrome
    • Inflammatory: common in ulcerative colitis
  • Neoplastic:
    • Tubular adenoma
    • Villous adenoma
    • Mixed tubulovillous
    • Sessile serrated

Prevalence of colon polyps

  • Colon polyps are found in 15%–40% of adults.
  • Most neoplastic colon polyps are adenomas (approximately ⅔):
    • 30%–50% of individuals with 1 adenoma ≥ 1 other adenoma.
    • Less common in Asia, but increasing as Western diets are adopted.
  • Most colorectal cancers (CRCs) arise from sporadic adenomatous polyps:
    • Early detection of adenomatous polyps by screening decreases the risk of CRC.
    • Lifetime incidence (in individuals with average risk) of CRC in the United States: about 4%.

Risk factors

  • Increasing age is a risk factor for developing adenomas.
  • Diet: high in red meat and fat, low in fiber
  • Inflammatory bowel disease:
    • Ulcerative colitis
    • Crohn’s disease
  • Family history
  • Tobacco and alcohol use
  • Obesity
  • African heritage
  • Uncontrolled type 2 diabetes

Hereditary polyposis syndromes

Each syndrome occurs in < 1% of the population and accounts for 5%–10% of CRCs.

  • Hereditary nonpolyposis colorectal cancer (HNPCC, or Lynch syndrome):
    • Most common inherited cause of cancer 
    • Accounts for 2%–4% of CRCs
    • Autosomal dominant inheritance
    • Prevalence: 1 in 279 people
    • DNA mismatch repair (MMR) gene mutations
    • Cancer occurs at multiple sites.
    • Slightly higher number of neoplastic polyps than normal; also, higher rate of progression to CRC
    • Associated cancers in individuals with HNPCC and/or in 1st-degree relatives, especially endometrial, ovarian
  • Familial adenomatous polyposis (FAP) syndrome and variants: 
    • Autosomal dominant inheritance or sporadic mutations
    • Gene affected: APC
    • Classic FAP: prevalence 1 in 7000 people; each has ≥ 100 polyps.
    • Attenuated FAP: a mild form of FAP (10–99 polyps) 
  • Juvenile polyposis syndrome:
    • 1st polyps appear at < 5 years of age
    • Autosomal dominant
  • Peutz-Jeghers syndrome (PJS)
    • Presents in childhood
    • Autosomal dominant 
Colorectal polyp types

Colorectal polyp types, approximate relative incidence (I), and malignancy risk (M) (2019)

Image: “Pie chart of colorectal polyp etiologies” by Mikael Häggström. License: CC0 1.0

Pathophysiology

Benign polyps

  • Hyperplastic polyps: benign reactive epithelial proliferation
  • Hamartomatous polyps: may occur sporadically or be genetically determined
    • Have different proportions and placement of normal colonic tissues
    • Cowden syndrome: mostly in PTEN hamartoma tumor syndrome group 
    • Cronkhite-Canada syndrome: nonhereditary; unknown cause
    • Tuberous sclerosis: TSC1 (hamartin, TSC (tuberin), mTOR pathway)
    • Juvenile polyposis: sporadic or autosomal dominant, with mutations in SMAD4, BMPR1A; transforming growth factor β 
    • PJS: autosomal dominant
  • Inflammatory polyps: due to chronic cycles of injury and healing (e.g., inflammatory bowel disease)

Neoplastic polyps

  • Adenomatous polyp: low-grade dysplasia
    • May transform to high-grade dysplasia/carcinoma in situ (over several years to decades) and eventually to colon cancer as it gathers more mutations 
    • Precursors to the majority of colon cancers
    • Usually follow the chromosomal instability (APC mutation) pathway
    • Can have tubular, villous, or tubulovillous architecture
    • Correlation with invasive CRC increases with:
      • Size (most important factor): rare if adenoma < 1 cm; 40% of adenomas > 4 cm contain foci of invasive cancer.
      • High-grade dysplasia
      • Villous architecture
    • Initial mutation event causing adenoma/dysplasia: more common in regenerating epithelium (e.g., inflammatory bowel disease)
  • Sessile serrated adenomas:
    • 15% of sporadic CRCs
    • No cytologic features of dysplasia as in other adenomas, but similar malignant potential
  • Hereditary polyposis syndromes with neoplastic polyps:
    • Lynch syndrome: mutations in MMR genes
    • FAP and variants: initial mutation in APC gene
Adenoma-carcinoma sequence

Adenoma–carcinoma sequence from normal colon to carcinoma:
Colorectal cancer (CRC) formation begins with APC gene mutation (inherited or acquired) and methylation abnormalities. Other changes can include KRAS gene mutation. Late in the process, p53 deletion, loss of heterozygosity (LOH) at 18q21 (involving SMAD2 and SMAD4), with overexpression of COX-2 can contribute to further growth and progression to cancer. The accumulation of mutations, rather than the timing of occurrence, is most crucial in carcinogenesis.

Image by Lecturio. License: CC BY-NC-SA 4.0

Clinical Presentation

Most colon polyps are asymptomatic and discovered on routine colon cancer screening.

  • Symptomatic polyps may present with:
    • Blood or mucus in stool
    • Change in bowel movements:
      • Constipation
      • Diarrhea
    • Obstruction or intussusception if polyp large enough
  • Hereditary syndromes:
    • Lynch syndrome:
      • Present with a higher number of polyps than in the general population
      • CRC may develop without first forming a visible precursor polyp.
      • Typically, have sessile serrated adenomas in the right colon
    • FAP:
      • Positive family history of the disease
      • Present with CRC early, at 20–50 years of age (median age: 39 years)
      • Have ≥ 100 (or > 1000) colorectal polyps 
    • PJS:
      • May present with hamartomatous polyps elsewhere (stomach and small bowel)
      • Pigmented skin lesions: melanotic macules of the skin and mucous membranes
    • Cowden syndrome:
      • Presents with multiple hamartomas in the colon
      • Facial mucocutaneous lesions 

Diagnosis and Management

The definitive diagnosis and treatment of colon polyps are achieved by endoscopic removal of the entire polyp.

Diagnosis

  • Screening colonoscopy or other forms of colon cancer screening to detect polyps early:
    • Starting at age 50 years in people of average risk:
      • Age 45 in African Americans
      • Earlier if 1st-degree family member has CRC at age < 60 years
    • Earlier and more frequently in individuals at higher risk:
      • Inflammatory bowel disease
      • Hereditary at-risk syndromes 
  • Colonoscopy is needed for polyp removal if detected by other studies:
    • CT colonography
    • Sigmoidoscopy
    • Positive stool testing for occult blood and DNA mutations
  • Baseline colonoscopy findings:
    • No adenomas: 3.3%
    • 1–2 adenomas < 10 mm: 4.9%
    • High-risk adenoma: 17.3%
  • Molecular testing indicated in suspected hereditary polyposis or CRC syndromes

Management

Follow-up guidelines after colonoscopy and polypectomy (based on 2020 Consensus Update by the U.S. Multi-Society Task Force on Colorectal Cancer):

  • Based on the findings at 1st colonoscopy:
    • Risk of CRC in the 10 years following a normal colonoscopy: < 0.5%, based on findings in 4.3 million individuals
    • Quality assumptions: 
      • Complete exam to cecum
      • Adequate prep
      • Skilled colonoscopist with adequate adenoma detection rate 
  • Normal colonoscopy → repeat in 10 years
    • No adenomatous polyps
    • Hyperplastic polyps < 10 mm
  • Low-risk adenomas → repeat in 7–10 years
    • Number: only 1 or 2 found
    • Size: < 10 mm
    • Histology: low-grade dysplasia
  • Sessile serrated polyps (SSPs) → repeat in 5–10 years
    • 1–2 SSPs
    • Size: < 10 mm 
  • Abnormal findings below → repeat in 3–5 years
    • > 3 adenomatous polyps < 10 mm
    • 3–4 SSPs < 10 mm
    • Hyperplastic polyp > 10 mm
  • Advanced neoplasia or high-risk adenomas → repeat in 3 years
    • 5–10 adenomas
    • 5–10 SSPs
    • Adenoma or SSP > 10 mm
    • Histology:
      • Tubulovillous or villous
      • High-grade dysplasia
  • > 10 adenomas → repeat in 1 year
  • Segmental colonic resection for large (> 2 cm) polyps or lymphovascular invasion
Segment of resected colon in the unfixed state containing an invasive colorectal carcinoma and two adenomatous polyps

Segment of resected colon in the unfixed state containing an invasive colorectal carcinoma and 2 adenomatous polyps

Image: “Colon cancer” by Emmanuelm. License: CC BY 3.0

References

  1. Kumar, V., Abbas, A. K., Aster, J.C. (Eds.). (2020). The gastrointestinal tract. In: Robbins & Cotran Pathologic Basis of Disease, 10th ed. Elsevier, pp. 807–812.
  2. Nguyen, L. H., Goel, A., Chung, D. C. (2020). Pathways of colorectal carcinogenesis. Gastroenterology 158:291–302. https://doi.org/10.1053/j.gastro.2019.08.059
  3. Grover, S., Stoffel, E. (2020). MUTYH-associated polyposis. UpToDate. Retrieved November 12, 2020, from https://www.uptodate.com/contents/mutyh-associated-polyposis
  4. Macrae, F.A. (2021). Overview of colon polyps. UpToDate. Retrieved July 20, 2021,from https://www.uptodate.com/contents/overview-of-colon-polyps
  5. Peltomäki, P., Olkinuora, A., Nieminen, T. T. (2020). Updates in the field of hereditary nonpolyposis colorectal cancer. Expert Review of Gastroenterology & Hepatology 14:707–720. https://doi.org/10.1080/17474124.2020.1782187
  6. Gupta, S., et al. (2020). Recommendations for follow-up after colonoscopy and polypectomy: a consensus update by the US multi-society task force on colorectal cancer. Retrieved July 20, 2021, from https://www.asge.org/docs/default-source/guidelines/recommendations-for-follow-up-after-colonoscopy-and-polypectomy-a-consensus-update-by-the-us-multi-society-task-force-on-colorectal-cancer

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