Epithelioid Subtype Gastrointestinal Stromal Tumors of Stomach in an Endoscopic Biopsy: A Potential Diagnostic Pitfall.

Background: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract with a broad morphological spectrum. Although epithelioid GISTs account for 20% of GISTs, their morphological features may pose a diagnostic pitfall for pathologists due to their morphological similarities to poorly differentiated adenocarcinoma and lymphoma. Case Presentation: Herein, we report a 65-year-old male patient with gastric epithelioid GIST misdiagnosed as adenocarcinoma for four years. During this period, he was treated with chemotherapy combined with PD-L1 immunotherapy. The clinicians thought the treatments were effective. However, there was no significant change in tumor size. The patient's clinical symptoms did not improve significantly as well. Finally, an endoscopic biopsy was performed again and gastric epithelioid GIST was confirmed in our hospital through morphology, immunohistochemistry, and whole-genome sequencing. Conclusion: A broad morphological spectrum and diverse immunophenotypic changes of GISTs could represent a pitfall for pathologists. However, predisposed anatomical sites, morphology, and corresponding immunohistochemical markers are of great significance for the diagnosis of GISTs and the differential diagnosis from other diseases. On the other hand, clinicians should diagnose and comprehensively evaluate treatment effects based on the patient's clinical symptoms and relevant laboratory examinations, instead of over-reliance on pathological diagnosis.