Table 2.

MSI: recommendations and comments from the working group.

Recommendations
A) The first method for MSI testing is represented by MMR IHC, a widely available laboratory test, utilizing antibodies against MLH1, MSH2, MSH6, and PMS2. B) MSI-PCR-based molecular testing is indicated in case of indeterminate IHC results (disagreement or interpretative difficulties). The five poly-A panel is the recommended panel given its higher sensitivity and specificity. C) As a novel alternative tool for MSI testing, NGS should be carried out only in selected centers experienced in these techniques.
Comments
- Use the IHC approach for detecting the four MMR proteins and assessing MMRd in any sporadic cancer type belonging to the spectrum of cancers found in Lynch Syndrome, so including EC. - Standardize pre-analytical and analytical protocol of testing - IHC can be performed on biopsies or surgical specimens if available, preferring the best-preserved sample as first choice
The main advantages of performing IHC on biopsies are the following:
(i) the better degree of fixation of biopsies (ii) the early knowledge of MSI status in a pre-operative setting The main advantages of performing IHC on surgical samples are the following: (i) larger amount of tumoral representative tissue; (ii) the possibility to select the best specimen for IHC testing; (iii) the possibility to overcome tumor heterogeneity. - The presence of an internal positive control is mandatory for interpretation of results. - Move to MSI-PCR or NGS (in selected centers) as a confirmatory test or whenever there is any doubt in IHC interpretation. In particular, in the following events: - Indeterminate/equivocal/ambiguous IHC results - False-negative MMR immunostainings mainly caused by pre-analytical poor tissue fixation - Aberrant staining patterns such as cytoplasmic, dot-like, or perinuclear staining – Loss of only one heterodimer subunit (e.g., only MLH1 or only PMS2 and not both)