Colon tumor
Familial polyposis syndromes of colon
Familial adenomatous polyposis of colon, Attenuated FAP (AFAP)

Author: Jennifer Findeis-Hosey, M.D. (see Authors page)
Editor: Raul Gonzalez, M.D.

Revised: 2 January 2017, last major update February 2016

Copyright: (c) 2002-2017,, Inc.

PubMed Search: "Familial adenomatous polyposis" attenuated AFAP
Cite this page: Familial adenomatous polyposis of colon, Attenuated FAP (AFAP). website. Accessed July 17th, 2018.
Definition / general
  • Subtype of familial adenomatous polyposis (FAP) characterized by fewer than 100 adenomatous colorectal polyps
  • Colonic adenomatous polyps have high risk for progression to colorectal adenocarcinoma (69% cumulative risk by age 80) (Gastroenterology 2004;127:444)
Essential features
  • FAP variant characterized by markedly fewer polyps
  • Due to a defect in APC (5q21)
  • Patients might not require colectomy
  • Phenotypically and genetically heterogeneous (Gut 2006;55:1440)
  • Cancers usually develop at age 50 - 55 years, 15 years later than classic FAP
  • Patients may develop gastric fundic gland polyps, gastric or small bowel flat adenomas and gastric / duodenal carcinomas (Cancer 1993;71:2709)
  • Extracolonic manifestations of classical FAP are less common in attenuated FAP
  • Classically reserved for patients with fewer than 100 colonic adenomatous polyps but exact diagnostic criteria have not been firmly established
Case reports
  • Patients with fewer than 20 - 30 polyps may not require total colectomy if they have frequent surveillance colonscopies with polypectomies
Gross description
  • Polyps are usually more proximal (i.e., right sided) than in classic FAP
    • Rectum often spared
  • Polyps are flat, slightly raised or plaque-like
  • May have minute central depression or umbilication
Microscopic (histologic) description
  • Adenomatous polyps are microscopically similar to sporadic type adenomas
Microscopic (histologic) images

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Colonic tubular adenoma

Molecular / cytogenetics description
  • Associated with pathogenic variants in the 5' and distal 3' end of APC, as well as interstitial deletions of 5q22, which include the APC gene

Differential diagnosis