Genetics of Colorectal Cancer (PDQ®): Genetics - Health Professional Information [NCI] - Major Genetic Syndromes
Table 10. Clinical Practice Guidelines for Colon Surveillance of BiallelicMYH-Associated Polyposis (MAP) continued...
Oligopolyposis is a popular term used to describe the clinical presentation of a polyp count or burden that is greater than anticipated in the course of screening in average-risk patients but that falls short of the requirement for a diagnosis of FAP. Thus, oligo-, Greek for few, can mean different things to different observers. While conceding a lack of consensus on the matter, the National Comprehensive Cancer Network (NCCN) committee on CRC screening suggests an AFAP diagnosis is worth considering when 10 to 100 adenomas are present. It will be used here to describe the circumstance in which the polyp count (generally adenoma) is large enough, with or without any attendant family history, to raise in the mind of the endoscopist the possibility of an inherited susceptibility.
In the setting of known or suspected LS, the detection of one to ten adenomas is still in keeping with the diagnosis. A similar adenoma count in a young patient undergoing colonoscopy for symptoms or in a screening patient over age 50 years could raise the question of LS. In the appropriate clinical setting-early onset and positive family history-the detection of any number of adenomas may support the testing and diagnosis of a patient for underlying LS mutations, consistent with guidelines such as those offered by the NCCN. Some controversy exists over the utility of testing adenoma tissue for microsatellite instability (MSI), as the yield is lower than in invasive cancer. In general, and subject to the above caveats, LS is not routinely considered in a discussion of oligopolyposis.
One study considered a series of polyps (37 adenomas) from 21 patients with known MMR mutations, performing MSI and immunohistochemistry (IHC) for MMR protein expression. Overall, MSI-high (MSI-H) was seen in 41% and in 100% of adenomas larger than 1 cm. Adenomas measuring smaller than 1 cm yielded MSI about 30% of the time. Correlation between MSI and loss of staining on IHC was fairly high, although the discordance rate (17%) was higher than in other series that evaluated invasive cancers from known MMR mutation carriers. A higher MSI likelihood was observed in subjects older than 50 years. IHC staining in relation to mutation showed 8 of 12 MLH1 adenomas to have lost protein expression, with 10 of 20 adenomas from MSH2 patients to have loss of expression. In contrast, none (0 of 6) of the adenomas from MSH6 mutation carriers had loss of associated protein expression. The authors concluded that while normal MSI/IHC was simply not informative, abnormal MSI/IHC was as likely in larger (>8 mm) polyps as in cancers and thus a reasonable test to consider.