Unusual pattern of microsatellite instability in gastric neuroendocrine tumor

Abstract

Abstract Introduction/Objective Evaluation of microsatellite instability (MSI) is routinely performed in gastrointestinal tract adenocarcinomas including gastric adenocarcinoma. It confers a good prognosis and is predictive of response to immunotherapy. However, the role of MSI in prognosis and treatment of well differentiated neuroendocrine tumors is controversial. Accordingly, MSI status is not extensively evaluated in neuroendocrine tumors. Limited available literature reports the usual paired losses and loss of MSH6 only in gastric well differentiated neuroendocrine tumor is unheard of. The purpose of this case is to report loss of MSH6 only in a patient with gastric well differentiated neuroendocrine tumor, grade 2. Methods/Case Report A 64-year-old female presented with abdominal pain and nausea. Endoscopy revealed erythematous nodules in the stomach which were removed by endoscopic mucosal resection. Microscopically, the tumor had vague nested appearance with areas more infiltrative appearance. However, cytologically tumor cells were bland and monotonous. Immunohistochemical stains showed the tumor cells to be positive for synaptophysin, chromogranin, pancytokeratin, and Cam 5.2, and negative for CK 7, CK 20 and CDX2. Ki-67 proliferation index was 3.7%. Therefore, diagnosis of well differentiated neuroendocrine tumor, grade 2 was rendered. Additionally, MSI by immunohistochemistry was ordered which showed intact nuclear expression of MSH2, MLH1 and PMS2 but loss of nuclear expression for MSH6. Results (if a Case Study enter NA) NA Conclusion This case highlights the fact that MSI in gastric neuroendocrine tumors can manifest as only MSH6 loss. Whether or not, it is of clinical and prognostic utility, is still up for debate.