Abstract 511P
Background
Microsatellite instability (MSI) & mismatch repair deficiency (dMMR) testing can provide both prognostic and predictive information in early as well as advanced gastrointestinal (GI) cancers. Testing patterns have evolved in recent years with the advent of tissue-agnostic approvals for immune-checkpoint-inhibitors (ICI) in MSI-High cancers.
Methods
In this retrospective observation study, we analyzed testing requests for MSI status at our hospital between Jan'20 & Dec'23. The primary objective was to describe the testing practice for MSI status (proportion of MSI-H/dMMR, GI site, clinical stage) & the secondary objective was overall survival (OS) for MSI-H GI cancers. We excluded patients with biliary tract cancer. All patients underwent immunohistochemistry (IHC) testing for lack of expression of MMR proteins on paraffin-embedded tumor tissue & when feasible additional polymerase chain reaction (PCR) based assay was also done.
Results
Two hundred and ninety-nine patients were tested for MSI in the study period. The median age of the cohort was 57 years (SD 13.0, range 22 - 86) and the majority were male (70.9%). The most common GI site was the colon (69.9%) followed by gastric (27.8%) & small intestine (1%). Two-thirds (66.6%) of the tested cohort were metastatic. All patients were tested with IHC (100%) & additional PCR panel/NGS testing was done for 17.7%. MSI-H/dMMR was 13.4% (40/299) in the entire cohort, 24% (24/100) in the early stage & 8% (16/199) in the metastatic cancers (p < 0.00). At the data cutoff, the median OS was not reached in early-stage cancers. In the metastatic cohort, the median OS was 27.1m (CI 22.4 - 31.8) and was numerically superior for the MSI-H at 35.9m (CI 23 - 48) compared to 26.8m (CI 21.8 - 31.7) in MSI- Low tumors (p - 0.38). Among all eligible MSI-H cancers, ICIs were used in only 31.2% (5/16). In metastatic cancers, the biologic/targeted therapy was used in 24.6% (49/199).
Conclusions
Our study reiterates the differential incidence of MSI-H status based on stage, with a higher proportion of early-stage GI cancers being MSI-H. Less than a third of eligible patients received ICI. Improving access to ICI can lead to better outcomes for these patients.
Legal entity responsible for the study
The authors.
Funding
Has not received any funding.
Disclosure
All authors have declared no conflicts of interest.