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Genetics of Colorectal Cancer (PDQ®): Genetics - Health Professional Information [NCI] - Major Genetic Syndromes

Table 8. Clinical Practice Guidelines for Diagnosis and Colon Surveillance of Familial Adenomatous Polyposis (FAP) continued...

APC gene testing is an important component of the evaluation of patients suspected of having AFAP.[181] It has been recommended that the management of AFAP patients include colonoscopy rather than flexible sigmoidoscopy because the adenomas can be predominantly right-sided.[181] The role for and timing of risk-reducing colectomy in AFAP is controversial.[182] If germline APC mutation testing is negative in suspected AFAP individuals, genetic testing for MYH mutations may be warranted.[132]

Patients found to have an unusually or unacceptably high adenoma count at an age-appropriate colonoscopy pose a differential diagnostic challenge.[183,184] In the absence of family history of similarly affected relatives, the differential diagnosis may include AFAP (including MAP), LS, or an otherwise unclassified sporadic or genetic problem. A careful family history may implicate AFAP or LS.

Table 9 summarizes the clinical practice guidelines from different professional societies regarding surveillance of AFAP.

Table 9. Clinical Practice Guidelines for Colon Surveillance of Attenuated Familial Adenomatous Polyposis (AFAP)

OrganizationConditionScreening MethodScreening FrequencyAge Screening InitiatedComment
IPPA = ileal pouch-anal anastomosis; IRA = ileorectal anastomosis; NCCN = National Comprehensive Cancer Network.
a Fewer than 20 adenomas that are each <1 cm in diameter and without advanced histology so that colonoscopy with polypectomy can be used to effectively eliminate the polyps.
Europe Mallorca Group (2008)[185]AFAPColonoscopyEvery 2 y; every 1 y if adenomas are detected18-20 y
NCCN (2014)[92]Personal history of AFAP with small adenoma burdenaColonoscopyEvery 1-2 yIf patient had colectomy with IRA due to significant polyposis not manageable with polypectomy, endoscopic evaluation every 6-12 mo depending on polyp burden.
Colectomy and IRA may be considered in patients aged ≥21 y
NCCN (2014)[92]Personal history of AFAP with significant polyposisNot applicableNot applicableNot applicableColectomy with IRA preferred. Consider proctocolectomy with IPAA if dense rectal polyposis.
NCCN (2014)[92]Unaffected, at-risk family member; family mutation unknown;APCmutation status unknown or positiveColonoscopyEvery 2-3 yLate teens

MYH-Associated Polyposis (MAP)

MAP is an autosomal recessive inherited polyposis syndrome. The MYH gene was first identified in 2002 in three siblings with multiple colonic adenomas and CRC but no APC mutation.[137] MAP has a broad clinical spectrum. Most often it resembles the clinical picture of AFAP, but it has been reported in individuals with phenotypic resemblance to classical FAP and LS.[186] MAP patients tend to develop fewer adenomas at a later age than patients with APC mutations [135,187] and also carry a high risk of CRC (35%-63%).[5,188] A 2012 study of colorectal adenoma burden in 7,225 individuals reported a prevalence of biallelic MYH mutations of 4% (95% confidence interval [CI], 3%-5%) among those with 10 to 19 adenomas, 7% (95% CI, 6%-8%) among those with 20 to 99 adenomas, and 7% (95% CI, 6%-8%) among those with 100 to 999 adenomas.[189] This broad clinical presentation results from the MYH gene's ability to cause disease in its homozygous or compound heterozygous forms. Based on studies from multiple FAP registries, approximately 7% to 19% of patients with a FAP phenotype and without a detectable APC germline mutation carry biallelic mutations in the MYH gene.[5,135,190,191]

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