Appendiceal Goblet Cell Adenocarcinoma With Mismatch Repair Deficiency and Microsatellite Instability-High Status: A Novel Molecular Signature Guiding Immuno-Oncology Strategy.

Appendiceal goblet cell adenocarcinoma (GCA) is a rare amphicrine neoplasm, which recent studies have characterized as mismatch repair proficient and microsatellite stable. A 49-year-old male presented to our hospital with a Borrmann type 3-like ileocecal tumor extending continuously from a markedly swollen appendix. The patient underwent ileocecal resection. Pathologically, the tumor was diagnosed as high-grade appendiceal GCA, accompanied by lymphocytic infiltration, staged as pStage IVB. Immunohistochemistry revealed loss of MSH2 and MSH6 expression, indicating mismatch repair deficiency (dMMR), and molecular analysis confirmed microsatellite instability (MSI)-high status. Postoperatively, the patient was treated with chemotherapy, including immune checkpoint inhibitors as second-line therapy, which provided temporary relief of duodenal stenosis caused by tumor metastasis. However, the patient died of disease progression 37 months after surgery. To our knowledge, this is the first report of appendiceal GCA with dMMR and MSI-high status. Our findings underscore the necessity of incorporating MMR/MSI testing into the diagnostic workup of rare tumors such as GCA, particularly in young patients or those with a family history suggestive of Lynch syndrome, to optimize personalized treatment strategies.