# Case Report: A case of synchronous multiple early gastric cancer with a microsatellite instability-high phenotype

## TL;DR

This case report describes a rare instance of multiple early gastric cancers with a specific genetic instability, highlighting the need for molecular testing and potential immunotherapy.

## Contribution

The report presents a rare case of SMEGC with an MSI-H phenotype and discusses its implications for treatment and molecular diagnostics.

## Key findings

- The patient had three early gastric cancers with an MSI-H phenotype and BRAF mutations.
- The tumors were not linked to Lynch syndrome, suggesting a different molecular origin.
- The case supports the use of immunotherapy for MSI-H gastric cancers.

## Abstract

Synchronous multiple early gastric cancer (SMEGC) is a relatively uncommon variant of early gastric cancer (EGC). In this report, we present a case of SMEGC accompanied by a microsatellite instability-high (MSI-H) phenotype. The patient was a 69-year-old man who presented to our hospital with abdominal pain. The endoscopic examination revealed two lesions. Both lesions were pathologically confirmed as EGC, then the patient subsequently underwent endoscopic submucosal dissection (ESD). Nine months post-procedure, the patient returned with recurrent abdominal pain, leading to the diagnosis of a new EGC. Immunohistochemical analysis demonstrated that all lesions exhibited an MSI-H phenotype and BRAF mutant expression, suggesting that these lesions are not associated with Lynch syndrome-related EGC. The case was ultimately diagnosed as SMEGC with an MSI-H phenotype. The current evidence and clinical experience suggest that patients with advanced MSI-H are likely to benefit from immunotherapy and should be considered for early systemic treatment with immunotherapy as a central component. At present, research studies on the molecular characteristics of SMEGC are limited, underscoring the importance of conducting comprehensive molecular diagnostics of each EGC patient, which could help clinicians thoroughly understand the lesion’s characteristics.